I know what your talking about, that sucks. Is there any of the stud sticking up to get vice grips on at all? If so try a little heat and then try to unscrew it. About the only other thing to attempt is to drill the stud out with a small drill bit, be very careful however, the stud is hardened and of course much harder than the aluminum. If this is succesful you could get a small helicoil and finish the job up.
Good luck!!!