Make the dock an unproductive crappie spot and people will stop fishing it. If the dock is loaded with brush or a large swim deck and sits over deep water then it will draw crappie anglers like flies on a pile of crap. I would just put up a sign that says "Please do not leave jigs stuck on the dock.". This means that you will have to allow people to get out of the boat and grab their jigs though. I doubt many fishermen are going to bother anything on the dock anyways if they are just their to fish. I'm sure they would rather not lose their jig either.