as was mentioned earlier,, on your Home page, or Profile page, you select Customize Profile, and on the left side of the next page, you will see a long menu list...

about halfway down that is Edit Options... select that and you will end up on the page where you customize your availability to other members , n stuff,,,

about halfway down, (under "Display Options"), you'll see a box with Show Signatures, UNcheck that, and POOF,, all those pesky signatures vanish~!!