You shouldn't have to delete the team every time. When you click on the team, there is a "forfeit team out" option under the administrator tab. This makes all of their next games a forfeit and disallows them from playing in the league.

At UDMercy we do something like that. I have registration open the entire semester. So teams can sign up on IMleagues. I also allow teams to play if they walk up. I don't usually have the brackets set up until the time of the tournament, so will usually either do them by hand or take several minutes and add the teams via IMleagues. I have found that it works pretty well, but the downfall is that if you don't put them into IMleagues your participation number will be off.