Keep the quit ban, but advertise it more. Like if you select the quit button it will warn you that you might be banned.
On top of that, give a req point penality. Like take away reqs or put them in the negative.
Both great ideas, but especially the second one. Some players do not care about Req points, but I also like the warning system. There already is a warning system, but make it more apparent (somehow) and keep adding time to bans if the player keeps quitting.