I was thinking, when people change names, they leave their old account behind, which will also be registered, maybe a command could be built that unregisters your current account after you have transfered your account.
Example
Player1: !send Player2 2000
Player2: Ok, go change your name.
Player1: Brb
...
Gangster1 has joined the game
Player2: !send gang 2000
Gangster1: Thanks
...
Player1 has joined the game
Player: !unregaccount