Can you stop your neighbor feeding seagulls?

It is not illegal to feed seagulls. However, one might argue, if vast flocks of seagulls are being attracted to your lawn because you neighbor is feeding them, that your neighbor is causing a nuisance. I think the first thing to try is to ask your neighbor to go down to the beach and feed the seagulls there, where it will not be a nuisance. Other than that, see if there is anything applicable in your municipal by-laws.