Maybe you could print out the pics and post them on the front door of the gym with a note reminding them that the handicapped spots are for the handicapped.

If that doesn't work you could send them to the local P.D. Just make sure you get a good pic of the license plate.

Or you could leave a note letting them know that you'd hate to accidentally bump into their bike and knock it over but if they insist on parking illegally it's possible that it might just happen.