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Heat up two small brad nails and poke them into the plastic near the threads (not down the middle so you don't press the valve). Let it cool and unscrew with a plier

The cap on the service port is usually plastic. You may be able to cut a groove in it to use a screwdriver to remove it. If that doesn't work you may want to try a left handed drill bit, which will grab and unscrew it, but be careful not to damage Schrader valve. If that doesn't work you will have a hole drilled in it, try a easy out. If that doesn't work, by then you can not have much of cap left in threads, you may be able to just pull the small pieces left out with a hook and pick set. It shouldn't be that hard to get out if it is plastic.