"True, I can't make you X. But if we are having problems and our family isn't working together then fun things like [whatever SUPER fun thing is coming up soon] don't happen. So ... decide what you really want to do here."
My son pulled the "you can't make me" line twice. Both times, I said similar to the above in a matter-of-fact manner and gave him time to respond. The first time, he didn't care because he didn't want to go to a certain kid's party anyway! The second time, he apologized and life moved on. Hasn't happened since. He just turned 8 though, so I guess there is still time
