Very annoying. The first problem here is that you probably cannot identify the offender because he obviously did not call for PPR.
There are very good reasons why PPR is a legal requirement for all unlicenced and non-public licenced fields.
Is there some way that landing aircraft can be identified when the field is unattended, maybe one of those motion detecting wildlife cameras would work and they are not expensive. Worth a try even if all you end up with is a few pictures of foxes. You may need more than one, a little experimenting is called for. A whole load cheaper than a fancy CCTV contraption.