Discouraging Gulls from nesting on rooftops
One deterrent that seems to work on flat roofs is to put up a series of parallel lines of wire cables or monofilament line (at least 50 lb test) across your roof -- 10 to 15 feet apart, about a foot above the roof. A line of cinder blocks could serve as temporary attachments for the line on either side of the roof, or attaching a series of posts to the sides of your roof. Gulls are very protective of nest sites (as you have found out), but if you can get them to move somewhere else they will most likely return to that new place in future years. If you have tried deterrents to no avail, then you could apply to the US Fish and Wildlife Service (regional office) for a permit to remove nests and eggs. This will also discourage re-nesting there, but you will not get a permit unless you have exhausted other deterrent methods.
Another thing you can try is a sound device designed to emit sonic or noises unpleasant to gulls. There are numerous companies that sell waterproof, pre-packaged systems that are ready to go. An Internet search on "nuisance bird products" will pull up a variety of options.