Here's a good idea to go with the teaming ban. I've seen Hive and MCSG use this back then and it works most of the time unless the teamers use skype or another 3rd-party chat system.
-Make solo games scramble player names and skins. That means players get a completely different name and a different skin. This is Hive/MCSG's way to combat teaming. Any games you play on solo still count on your records.
-Disable ALL chat during the solo game. No PM and no public chat. Attempting to chat will say, "You are currently in a solo game. Wait until the end of the game." This is to prevent players from talking to each other during a game, since in most cases it's to have illegal teams communicate with each other for supplies or form teams.
Now, what about /fmsg from friends on a different game? How are you going to talk to friends? The solution: have an inbox so that the recipient can read them when he's back from the solo game. The player can type /f inbox or similar to read any messages he/she received during the solo match.
-Big warning message at the start of any solo game. In big letters, all players are warned that teaming will result in a ban.
-Separate ban system to work with the current one. Instead of a ban from the server for 24 hours, make it so the teamer is banned for 7 days on SOLO GAMES ONLY (can still log on and play team mode, team-based games and PvP) on the first offense. Second offense = 7 day ban from the server. Third offense = permanent ban from all solo games (this includes other solo mode games on the server such as egg/sky wars, lucky islands, and SG) + an extended ban from the server (2 weeks?)