That is the colossally retarded idea that everyone else is bandying around. Do you realize how much worse that would make DPS queues? Queue fishers are almost exclusively tanks, because only tanks have no or nearly no queue times.
The Duty Finder should, under no circumstances, tell you whether the group is new or in-progress. You should have no way of knowing until you get inside. Upon getting into a new instance, if you leave, then you will eat a 30 minute penalty.
This small change will stop queue fishers instantly.