Yeah, what Marc said. I've done that before only to have a similar idiot (I wonder if it's the same guy?
) go right back to his own ideas of what he should be doing. Eventually it becomes pointless to worry about it -- as an earlier poster suggested when we had a guy like that I'd make sure I'd pair up with him for some less than interesting practice to get it out of the way. Then I'd avoid him like the plague when it was anything I really did want to practice sincerely.
There's a lifetime left to practice. Guys like that are just bumps on the road. Let sensei deal with it. If they don't, fine, don't waste more time than you need to. And certainly don't waste your mental energy worrying about it.