Right now tournament organizers cannot kick players from a fixed line-up once they joined. The only way to remove them is to kick them from the team completely, which is not ideal.
For example / recreate:
Step 1) User is inside a team
Step 2) Arena with fixed line-up name "XY" is created.
Step 3) "XY" joined the Arena.
Step 4) Tournament creator cannot remove "XY" after Step 3 anymore.