Trying to lock the user into the game by removing the leave button is a bad idea. Instead they should make it very clear that they aren’t supposed to leave: for example by making the button red/yellow/whatever with a warning sign and a warning message that they get penalised for that action.
Something I liked in CS:GO is that after searching for competitive it asked for pressing the READY button (to make sure people aren’t AFK after waiting for search) and they made it clear before the match that pressing READY means committing to a full match that might take half an hour and leaving is penalised.