How do you politely handle an extremely assertive vendor?

Send them an e-mail that says exactly what you put in your question. It sounds reasonable, encouraging about the possible future business between your companies, but firm about your expectation of respect and cooperation.

Good sales people are used to being ignored and only makes them work harder at a sale they know has big potential. I always let them know upfront that I welcome their information and will contact them if and when. If I really like the product but not the rep I let them know that as well and request a different sales rep.

Why not just say it as you did there?

I would say this in a face-to-face meeting.

“I want to clarify that I am the decisionmaker about whether we will be putting in your service. It does not help your cause (and in fact may hurt it) if you keep cc’ing everyone in the practice on your emails.”

I would then say, “Look, I appreciate your diligence, but I cannot always get back to you within the time frame you have allotted. You need to give me 48 hours (or 72 hrs or one week) before pinging me a second time.

“You need to find a way to accommodate the practice’s way of working so that we know you will fit in with our efforts here.”

“Do you have any questions?”

I agree with everything @marinelife said, except one thing. Don’t say “you need.” Say, “I want.”

Or, alternately, a formal letter with company letterhead;

Dear sir.

Regarding your recent attempts to secure a contract with my company, please note the following:
I am extremely interested in your product/services and am seriously considering utilizing said product/services.
I am keenly aware that you are anxious for an answer from me and I can assure that you that I am giving due consideration to this matter.
Due to other constraints being placed on my time, I am often unable to respond to messages within 24 hours. You can expect to hear from me within 72 hours at at the maximum. Additional messages are cumbersome and cause an extra backlog that increase your wait time. In addition, use of the cc feature is unnecessary, as I am the only one who will be making the decision in this matter.
Please allow me the courtesy of a 72 hour return time before sending additional messages, and do not cc others in the company regarding this matter as it is out of their purview.
I am looking forward to doing business with you.

Sincerely,
XXXX

Good luck!

I think you’re trying to equate “polite” with “inoffensive”. It may be that you will have to tell the emailer—in a blunt, but still polite and “businesslike” way—that his attempts (and be certain to individuate this to “him” to show him that he’s alone here) to intimidate you by emailing everyone in creation when you “fail” to respond to the seller’s unilateral decisions of “acceptable response” and “acceptable timeframe” for the response risk backfiring on him, and getting him blackballed simply for “unacceptable business practices”. Furthermore, if your friendly warning goes unheeded, then it is your option to take the case to his management and have him removed, replaced—or worse, if his management deems his conduct against your wishes to be flagrant enough to merit actual discipline or termination.

I would not be coy about this. But it’s perfectly acceptable and perfectly polite to be very clear that you are displeased by his insistence on “his way” all the time and by his blizzards of emails that attempt to circumvent your actual authority. It would certainly help if you have your management on board (and present) with you at this meeting, but that should not be completely necessary.

If he responds that these are his marching orders from his management, then obviously you do need to escalate your demand. It’s a perfectly reasonable demand.

I repeat the words,“No,Thank you.” until they get the hint.I’m stubborn and will outlast them ;)

Don’t be polite. Vendors take advantage of politeness.
You don’t have to be a dick but you need to take charge. If they don’t provide something you need, tell them to shove off. Vendors are time-killers.

I heard of a doctors office that said, “we only accept vendors on X day at X time and you must bring lunch.”
Tell them that you accept vendor appointments every other Thursday between 2 and 4. Tell them they are allowed 15 minuets to make their presentation. Make an appointment with them then set a timer. You will get a reputation with everyone for having good time management skills. It might even set you up for a promotion!

THANK YOU all, for such great and thorough responses. These are really helpful!

Good bye.

@CyanoticWasp has a good point about contacting this company’s management. They’d probably want to know that their salesperson’s aggressive and disrespectful behavior might lose them your business.

Thanks, @Haleth. Usually if I keep talking long enough in complete sentences I say something that makes sense. Or seems to, anyway.

“Dear…

I am the person who makes the decision about whether or not to add your services, I’m the person that you are going to have the business relationship with and that if we’re going to work together in the future, this behavior needs to stop.

Yours sincerely
...”

I tell such vendors politely that I am very busy, and as a way of managing my time, I have had to adopt this approach, which I apply equally to all.I will give them due consideration only if they play by my rules.

My rules are that they email me their input and wait patiently for me to respond if and when I am interested. I tell them that if they try to follow up with a string of emails, or phone calls or try to go behind my back or over my head, their original email will be removed and all consideration of using their services will cease.

Of course, you have to have the backing of those above you and even those a level or two below need to be aware of what you are doing and why. In my case, since I own the company, everyone I tell this to believes I mean it. :-)

Go over his head. Tell his manager that you really like the product, but his agressiveness is very off-putting, and you would prefer a different sales person be assigned to the acount, or consider it lost.

I’ve had to do this with printing reps in a past life. I have no problem with being direct about this. You are under no obligation to buy from anyone.