Hello DucaninDevon
I think Foxy Loxy had a good idea - i.e. when it is ATC tell them "G-XXXX, this is ZZ APPROACH, full ATC service - but it depends just how busy you are. An alternative might be to reverse Foxy's idea and tell them something like - "G-XXXX. this is ZZ Radio - Information Service Only."
Generally, in my experience, the friendly chat approach worked best so long as you can get the individual to either see you or 'phone in. However, please don't hesitate to report persistent offenders since it can (unfortunately) be the only way some people learn!
Presumably you have checked the entry in "Pooley's" - normally it is fine - but if you find a problem with the wording why not write to them and ask them to make it clearer?
Last thought - a little session with local pilots sometime might also help by getting them to spread the word.
Peatair