What do you think of backing out of the competition to punish my friends.
Why do you feel like you need to punish them?
They want to do this competition but some of them have been missing practice so I don't think they're committed.
I can understand you questioning their commitment to the competition but you shouldn't punish your friends. Just try your best and the punishment will be them not doing well if they haven't practiced enough.
What? That is so out of context and lame.
What I'm saying is that you should let natural consequences punish them instead of trying to get even with them yourself. Of course you risk losing a competition you've competed in, but if you back out you don't even get to compete. This way they may learn a lesson.