How about you can't leave until the round is in a join or leave state.
Well you can't force someone to stay in a server, it's not possible.
If when they join your server and a round has already been started, they would spawn in spectate mode. Once finished, or restarted, they could choose to enter, or not enter the next round. Almost seems like a tournament.
maybe once they die they get teleported to a room with an obstacle course or something to keep their patience?
Or maybe it'll allow them to build till the next round starts, with a limit that way they couldn't spam up the server. And once the round starts, it would delete all bricks that were not in a certain area, or something like that.