Try the product "goof off"..
I made a huge mistake also. Smeared right in the middle of a 24" X 36" installed pane. Tried soap and water: Nope! Tried Acetone: Nope! Gas: Nope! Goo Gone: Nope! Haven't tried Goof Off: Will try. Will also try NOVUS also. Let this be advice to any DO-IT-YOURSELFER: Plexiglas is great material, but you have to treat it very delicately. We need a chemical engineer to figure this out for us.