How do I say thanks, but no thanks?

I would just take the bull by the horns and tell him plainly “no”. It’s your endeavor, your project and perhaps he can do his own comic book design if he feels like contributing more. Perhaps he’s not getting the message because your answers are vaguely negative and not to the point. Or, he is just deciding to ignore and persist, hoping you’ll give in. But since this is your project, you have to draw the line (literally) and tell him where he comes in. This will be good practice for you to say no to people and be firm for the future when you’re running your own business. :)

This is your project, like @Siren said. Just suggest to him that it would be much more efficient and consistent if you were to take care of what you’d like, and that he’d be very appreciated as a plot writer. If all else fails, a straight up no.

You’re going to have to be firmer in your “no” if he’s not getting it. It’s your project.

Don’t forget to stick a “I’m sorry” in there :D

I would just tell him that to get your project up and running your greatest need will be for him to concentrate on writing. Tell him you need him to be your idea man and keep that offer within those boundaries.

Simple: push back. “No. Sorry, but no. I’m going to do the drawing to keep it all to one style consistent with my concept. You’re a great pal, but for this the answer is no.”

The less someone understand politeness, the thinner the layer of it you should paint on.

(He’s going to have a tough time in Japan if he has no concept of courtesy and subtlety.)

Tell him simply, “No, it is not going to work out for you to do art on this project.” Then don’t explain. Simply repeat.

If he’s not getting it already you’ll just have to plainly say no. You don’t have to say it’s because he’s not up to standard, but don’t start making up excuses or he’ll start trying to find ways round them.

In Borg?

Keep it positive. Write him an e-mail if you can’t say it out loud.

“Hi, Taglie…I’ve been thinking alot about the project and feel that I want to do the pencilling myself. You are still exploring your techique and it differs from mine. However, I still believe that your strength is in the fantastic story lines you come up with and I’d like to keep you as a consultant, maybe? So, what about collaborating that way? I would give you credit, too. Something like “Art by U. Bagel based on a story by Taglie Titchmarsh” or “Art by U.Bagel based on a concept by Taglie Titchmarsh” If he is giving you ideas, please give him credit for that or if you sell the comic, give him a bit of a cut, too. It’s only fair.

( I speak as someone who has helped scads of people with creative things/writing/ideas and rarely got credit till I put my foot down. Ideas are really worth something.)

Downtide is right. It sounds like you’ve already been polite. How about “I’d rather keep you on for plot ideas than as a penciller, so the answer is no.”

“Dear ____,
I think it’s great you want to be involved. I’m going to do the penciling, but I’d love to have you as a writer.”

Thank you everyone for the suggestions. Next time I talk to him I’ll bring it up, and tell him no firmly, but that I still want to keep him on as a plot consultant. ^_^