Get a thin flat blade screwdriver under the tee nut and gently lever it up. keep moving round the four quadrants. Once it's up enough you should be able to get some pliers on it to grip it while you unscrew the part connected to it.
Or cut the screw head off and drop it through. New t nuts and screws are cheap from pinball heaven/mania. Don't waste your time on them mate. Just cut the buggers and be done with it