One thing to remember about ANY living animal at Disney World is that you, as a human, are basically in a nature preserve. So, if a seagull or squirrel or ibis (those long legged white birds), ducks, rabbits, whatever, are around - Disney is not going to forceably remove them just because us humans think the animals are a bother. Yes, Disney will remove an alligator but only if over a certain length and then it's to put the alligator to another spot outside the normal property.
Are seagulls a bother? Certainly because they are doing what they need to do - get food. And us humans will give them food. Seagulls can't talk like we do but they can communicate and if "word" gets out that food is to be found, others will follow. Also, birds are habitual. If they know at a certain time during the year, they can get food a X location, they will go back to X location. And if Disney World is X location, they will be back. Simple solution is to not feed the birds or any other animal at Disney World.