We have the same problem with fixed wing pilots.
Some local flying instructors believe they can use our strip firstly without permission and secondly without paying a landing fee. The strip is often unmanned during the week. To add insult to injury we have received noise complaints associated with this activity.
We are investigating fitting cameras at both ends of the runway.
If the airfield is PPR then the landings are also illegal. If you can identify the culprit you can get the CAA to sort them out. They just love to prosecute G.A pilots.
ericferret