What you might try next time is one of those stamped steel sockets you find in the very cheapest tool kits or the spark plug wrenches that come with chain saws and such. But that’s next time. Right now, we need to fix this-
First thing I’d try is a piece of wood, say 1/4-3/8" or 6-8mm square and 150mm long or so, square cut ends, preferably a softwood like pine or spruce, and a light hammer. Not a screwdriver. Put one end against the side of one of those six tabs sticking up and tap away industriously at the other end of it with your hammer, see if you can get the plug to start turning. The softwood will have less likelihood of 'chiseling off ’ the tab than hardwood and far less than the screwdriver I told you not to use. You have six chances with that, if you bust off all six tabs and you have to -
Try an EZ-Out, tapered thing that looks like a machinist’s threading tap with one end square and a left hand thread, used by mechanics and machinists to remove broken bolts from tapped (threaded) holes. You turn it in counterclockwise and it cuts threads in the sides of the hole until it won’t go in any further or starts the plug out, then put some torque to it ( use a tap wrench on this, NOT pliers or the spanner that got you into this, the tap wrench gives you far better controlled 90 degree force) and with any luck that breaks it free and backs it out. DO NOT break off the EZ-Out, the steel in those is very hard and can be brittle and if that’s stuck in the hole you’re screwed, pun intended, you won’t be able to drill that out. Use gentle pressure. Know when to give up and try something else. With any luck that will get it - otherwise- -
Get a SHARP twist drill bit, 1 1/4" diameter, do not use a paddle bit or wood auger or Forstner-type bit or one of those stoopid stepped things, you should be able to find one with a 1/2" shank. The reason to use the twist drill is that it will tend to follow and stay centered on the hole that goes down the center of the plug. Drill out the whole assembly with low and slow drill RPMs and gentle pressure and replace it according to the makers directions. A dull drill or one run too fast will chatter and give you an oval, oversized hole. You don’t want that. Too much pressure, well, once you cut all the hard plastic you get to the foam, which is going to offer no resistance and then to the bottom of the board. You don’t want matching holes top and bottom.
You could try a sharp twist drill bit just under the size of the threads, say 10-11mm or so, to drill out most of the plug before trying to remove the whole assembly, then try to snap what’s left out by tapping on the top of it with a small flat head screwdriver and a light hammer, break off a side of the drilled plug and the rest should come out easy. Just be careful, if you bugger up the threads you will have to drill out the whole thing and replace it, as above.
And when you finally do replace the plug, one way or another, a little silicone grease on the threads will be your friend and with any luck you won’t have to do this again
hope that’s of use