At times you can use a SAE allen that may be just a little larger and get it to loosen, if that won't work the next step would be to drill it out untill the head just falls off, then you can use vice grips and remove the old screw, after you remove the head.
You will need a good drill bit just small enough to fit inside of the hex inside the allen screw. There tough but can be drilled out enough to pop off the head.
This one does not sound like it is in the open where a person can use the dremel to make a slot in it for a plain screw driver, thats another solution to use sometimes.