I am about to start making hand poured plastics myself and have not yet had to deal with this problem but has anyone tried using rubbing alcohol once you get through using the dye(s).
A couple of other suggetions might work are a can of WD-40 or a spray bottle of Goo Gone both of which can be picked up at Wal-Mart for a couple of bucks. I don't know how well my suggestions might be but if it helps out then let me know because like I said before I'm about to start this myself.