is the box ported?
if the box is ported it sounds like it may be 1 of 2 things when it was screwed in the peice thats meant to hold the screw came loose so its screwed in and just spinning
or its bolted and you will have to reach inside the port and hold the other side.
you need to find a Torx screwdriver. Look for it at sears, ace hardware, kragen, maybe pep boys. if it isn't torx, it might be something right around that area. talk to the worker there.
grab the speaker somehow and pull up on it (out of the hole) while you're turning the screw. this should make it come out. if they're all like that, you may have to saw the heads off.