I keep getting emails from an organization I was associated with years ago. They do not have an opt-out link. How do I get them to stop?

You have done the recommended first steps.
Call them at this point and state you need to be removed from whatever distribution you are on.
There is an RFC in the industry for email unsubscribe that defines it, but it may not apply to them or have implimented it.
Many of these emails come from addresses that are unmonitored or not a valid email back.
Call them and be a bit more stern, but the simple way to fix this is put the email address of the email you get on a block list or defined spam on your client. Do this for any email coming from the entire domain name @xyzcompany.com

I would call them at this point without thinking twice.

Welcome to Fluther.

Depending on your email client, you may be able to set up a filter to simply discard those emails as soon as they arrive.

I love gmail’s options for this. I throw away nearly half of my mail without even knowing that it ever arrived. (And that’s on top of their generally excellent spam filters, that winnow out… most of the other half. Sigh.)

Mark them as spam.

I got a call last night from a magazine that my daughter got in 3rd grade. She’s been out of grad school for two years. They called and the first thing they said was, “How’s ___ enjoying ___ magazine? Are you interested in renewing?” I replied that the last time she got the magazine was when she was 8. The person said, “What does her age have to do with anything?” I replied, “She’s 24 now.” “Oh.” Click.

I kept getting mail from the DNC and the GOP. The GOP stopped sending me stuff when I started making donations to the DNC in the GOP’s name.

DNC didn’t stop until I started packaging some bricks and attaching them to the postage prepaid return mailers, but not until I had almost cleaned up the construction debris from building my house.

I don’t know the answer but I hear you. I have changed jobs and still get people adding things to my caledar that relate to my previous job plus emails about things that have no relevancy to my present job. Frustrating. Keep letting them know, block where possible and siphon them off to the spam file where you can (i.e. using mail rules).

SPAM, SPAM, SPAM, SPAM

Block it.

I feel your pain.

You can either set it so it goes directly to your spam folder. Or you can call them directly and ask them nicely to stop emailing you. If companies have an internal do not call list then surely they must an opt out email list.