Final3 and Gorgeous - Take your points, but I trust the police will check back to see who was sitting in the seat and the seats around it for the last few flights.
The reason is the pax can be asked to submit handwriting tests. Handwriting experts can determine this sort of thing.
It may not work, but surely its worth a try? Convict one of these arseholes and sling him/her in the slammer for a while and I think this sort of nonsense would reduce dramatically.