Any way to LOCK role

Our serve is going to have a competition but first we need members to choose their role(group) so that everyone could earn scores and trophies for their group. At the end of the activity we will count scores for each group and one of them will be the winner, and we have already solve the counting scores problem.

Problem is members can change their role whatever they like anytime they want(in 'channels and roles'), causing us cannot get a balance number for each group.

We are looking for bot or solution to lock roles in order that nobody could change roles optionally. 

Really need help!