Thanks for the replies guys!
We have tried the "gentle reminder" route with the people concerned, but still they insist on coming. The old "it's in Class G Airspace, we can do what we want" attitude seems prevalent, hence why I thought maybe the reminder about Rule 5 would make them sit up and take notice.
All the aircraft at our strip operate on SAFETYCOM whilst on the ground and within 3nm, and we always inform the local Radar unit when we are active.
I guess our frustration is that despite our best efforts, and the promulgation of an AIC with regard to SAFETYCOM, no notice is being taken by seemingly professional instructors and qualified PPL's.
Above all else, it is just downright rude to use a private strip without the owners permission, except in a real emergency! We are not being exclusive, however as all strip owners know, noise abatement is a huge issue if we wish to continue to be able to keep the strip established.
One of these days, there is going to be an accident just because people "can't be bothered" to phone up before hand or get themselves used to using SAFETYCOM!
Cheers!