If jamming something into it like the tang of a small file doesn't get it to turn, use the cable slot to cut all the way through it from the side with a loose hacksaw blade. Take is easy on the far side, but you will cut a little bit into the threads. That won't matter. Give the part sticking out a little squeeze and try the screwdriver to turn it out.

Loosey lefty.

If you can't get it out in 20 minutes, spend $20 on some used levers.