I support this, however scammers are smarter than this.
It will be effective at first but later on you'll notice all the flaws in the system:
1. You'd have to know the scammer's real IP, it's possible that innoncent people get banned if they have the bad luck of chosing a username that matches the scammer's username on another forum. This is not likely to happen very often but it's possible.
2. Scammers will find out this rule has been instated and are usually smarter than this so they simply register under another name.
3. If they do get banned they can still claim to be innoncent by stating it was just a coincident and unless there is an ip provided by the other community he got banned in there will be no way to know 100% sure if it's him.
Basically, the main issue here is that you'd need the cooperation of the other communities' staff members to get his actual IP, or you can start using Skype resolvers even though those are not always very efficient and precise.