It is sometimes better to thru drill the joint & use a nut & bolt, rather than to tap a SS machine screw into aluminum, for just this reason.
Heat can be your friend here. Aluminum expands roughly twice as much as most steel alloys, when heated. Enough heat will often soften the bond between the aluminum & the ss so that you will be able to turn the stuck bolt out of the threads in the aluminum.
If you end up needing to drill SS, use a 135 degree split point bit (preferably Cobalt) & use some sort of flood coolant along with a slow drill speed. Real cutting fluids work best, but a steady stream of plain old water will get you by. Proper drill speeds for SS are roughly 1/4-1/5 as fast as you should use in mild carbon steel. Go slow & use plenty of coolant, so that you don't roast the bit.