Maybe set it for a few conditions. A couple suggestions would be.
A: No logins for av's under a certain age.
B: Only those with payment info on file
C: Only those with Payment Info used
D: no 1 day av's
It's usually people who are making brand new alts with no payment info that are doing the grid attacks. This way the legitimate population can still get in.
While it's not a perfect solution and some people would be excluded (not all new av's or no payment info people are bad), it would still allow quite a few into SL while trying to kepp the attacks from reocurring