Bet you were going to suggest a 12 bore works every time!
Although the sails and hull end up a bit worse for wear.
I have a good method that works well, scares all the gulls off of my boat and onto others. If I make it public knowledge then my advantage would be gone and I'd be back to square one.
I think that Iain's suggestion of CDs would work as good as anything. On my sailboat, I simply run a light 2mm line around the outside of the rigging just above head height. My theory on why it works is that the sea gulls are frightened of getting tangled in it. It has the advantage of not being too unsightly.
Best of luck.