[Leimi]

It's pretty simple. "they won't pay at all": would you want to add the feature if he paid you? Then tell him that.

Don't want the feature? Just say no, close the issue and move on. Especially if your project is MIT licensed, it's crystal clear in the license that very one thing: you don't owe anything to anyone :) You don't need to be nice, you don't need to explain. Life's too complicated to be upset about those things.

If the person keeps on insisting after that, you can block him from GitHub.