Get a few cheap u-locks at WalMart and put 'em around the front fork/spokes of the motorcycles.
If you want to be a complete prick about it (and I would,) get a small padlock and put it through the slot in the disc rotor. It will probably go unnoticed until the rider tries to roll his bike out and it ruins the front brake and he has to take a cab home.
-- Yeah, I've finally had it with people parking where they're not supposed to and a local PD that seems indifferent about it. Bike lanes, handicap spots, fire lanes, hydrant zones, etc. A guy zipped (illegally; no hang tag or plates) into the handicap spot my fiancee was going to pull into, and when I got out and yelled at him, his excuse was the standard "I'm just running in to drop this stuff off. I'll only be a minute." So I hocked a giant loogie on the hood of his car and told him I'll only spit on his car for as long as he leaves it parked illegally. I mean, it's not like I'm going to do it for a long time or anything, so that makes it OK, right?
__________________
"I feel like my world was classier before I found cyclocross."
- Mandi M.