You may ultimately have to drill it out.

Another thing I've done to get a better grip on the screw, is to take that wide screwdriver you've got, heat it up (not cherry red, just hot) with a propane torch, then quickly and firmly stab it into the screw. You'll then perhaps be able to get better leverage on the screw.

Drilling it out is no big deal, as it's obviously needing to be replaced anyway if you're having that kind of trouble.

The old On the Wire web site had a good article on cleaning up the threads after you get the old delrin screws out. All of their articles are now on thebeachcats.com, so you may want to look for that as well.

Cheers!

Tim