Crush it!
Morning Dave:Those things are relativily light weight material and there should be a washer behind the nut. You can probably get away with putting a pair of vise grips on it and crush the nut. If it's rusty it will probably break away fairly easily. If not, get your hands on a dremel tool with a small cutting wheel. The washer should protect your fiberglass, but I would work very slowly. Replacements can be had at just about any marine supplier like West Marine or Boats US. They are common.Key in thru-hull vent on the attached web site.