I have had the same problem with the nylon plug in my urchin...
Assuming that the screw head is still in shape so you can get a screwdriver in there, here is what I had to do when mine got stuck...I found the widest flat head screwdriver I had, inserted it in the groove on the top of the plug and I verrrry sloooowlllly turned it counterclockwise until the plug started to move. I was able to get it out but it really ripped the top of the plug all apart. HTH