A baned account still exists, it is not deleted but it can not be accessed... it can be attacked, altough you won't find anything but some segmetns on him right now.
He used a bug that was making a tiny fleet bigger, by forcing upgrades in the ship using posting scripts to bypass the checks. When a forced upgrade was induced, it triggered a protection system in place, that was trying to repair the fleet. The protection system made the ship bigger in space to fit the upgrade, not allowing the space to go negative. This had the side effect of making the fleet more expensive. By buying the fleet first, using the bug, and then selling the fleet, he would get more credits.
He was warned, the fleets where fixed, but the problem wasn't clear until he used it again. Then he was baned, the investigation revealed the problem, and now the system is fixed on this part.
The protecton system was upgraded, the fleet space will not be increased anymore, but if for some reason upgrades would be forced on a fleet, the system will start and check every upgrade, and remove the intruding upgrades on detection, leaving the clean fleet in place.
Also the fleet checker will start now and then on its own, fixing any undetected anomalies...
So in the end, the only thing he managed to do is to make the system smarter...
God Speed Commanders!