If you have registrations, a simple letter to the owner - address on G-INFO would at least get them to consider their actions in future.
We had a microlight guy flying very low around Harrogate a couple of years ago, I got the reg, wrote him a pleasant letter, he stopped.
Flip it the other way round, I departed Breighton some years ago during the week, at around 700', my passenger shouted - Jet ! A Tornado passed close enough for me to hear him and see the pilot.
Yes, he was legal, yes I was legal, but it begs the question of if we had hit, who would have been deemed to be in the right ?