Try this:
Take a 12 mm half-inch drive long nose socket, preferable a hexagonal one as opposed to the bi-hex design.
Hammer the square end on to the filler plug. Then insert a large 12 mm Allen key in the hexagon end and turn.
If you don't have such a big Allen key then a hex bit socket and driver will do.
You can use any matching socket and Allen key e.g 10mm socket and 10mm Allen key, I just chose 12mm because it gave a big enough Allen key to put enough torque on to undo the plug.