I once made a "rebellious" signature - to get the admin to put some signature restrictions on that site. It was a 5000 x 2000 transparent GIF image. It stretched out every forum page like mad, and put a huuuge space between my posts and the other's. Just 1kB in size, but more annoying than anything of this world.
It worked. The admin thing, that is. No hard feelings either, just laughs.