Why would you want to force anyone to do anything? If you made it clear that someone can only be platonic friends with you then he needs to respect your decision and either be platonic friends with you or leave you alone and find someone who is willing to be romantic with him.

I get wanting to be platonic friends with someone as well as liking said person's company, but if you know that this guy will press you to be more than friends, then you may need to fall back and leave him alone. I think you both can not be platonic friends ends unless the both of you agree to be platonic friends. Otherwise, there will be some conflict, even if it is subtle with no arguments.