sigmagrrl, I know exactly what you mean. I've seen more than one pledge class w/ the "you can't make me do anything, it's hazing" attitude. Anyway, not going there, time to help pikeks109.
Do you have a national pledge test on basic knowledge, et al? If they are as clueless as you say, they will flunk. They won't be able to be initiated due to NATIONAL STANDARDS and there won't be thing one the chapter can do about it.
Do brothers have to go to a certain %age of events to be in good standing? That applies to the pledges, too. If you don't have a system like this in place it might be time to get one.
Rather than the "shape up" lecture try reverse psychology. Ask if there's anything they feel they aren't getting from the brotherhood - ask if they are sorry they accepted their bids, since that's the impression they are giving. Tell them if they are that unhappy, they can leave.
But if you do ALL THAT and they still are a-holes, don't hesitate to give them the boot. You don't need that kind of cancer on the brotherhood for the sake of numbers.