From what you're saying, sounds like the shells are the problem moreso than the gulls themselves right? Why don't you guys get a leaf blower for the club and keep it in the toolshed. Take it and blow the runway down before a flying session.
Nuisance animals like seagulls always seem to adapt to anything you do like rubber snakes, airhorns and fake owls. Now, a real bird of prey would be effective... but then you're getting into an entirely new hobby.