I would make an Airprox report: indeed, did so three times in a year, if I remember right, here at Strathaven.
Local ATC units will then be able to ID the aircraft and you can then have a quiet word with the pilot. He may have had such fun he feels like trying it again, so important to spread the word. There is no "fine"or shame here.
Of course, the issue with an airfield is conspicuity. If marked on the chart as disused, then that is something to be changed. No need for the Airprox board to come up with that recommendation!
The microlight school may want to send out leaflets to local airfields? We have done that at Strathaven when we went from being an almost abandoned airfield to home to 32 aircraft.
ATZ: you can have one with an A/G service if you are licenced. Or you can become a FISO and run it unlicenced. Neither radio operators can tell anyone what to do once they enter a runway or are in the air.
So why this distinction? There is a lot to be done in Red Tape challenges!