In reply to JJL:
> You are completely right.
> Try to get the instructor away from clients and approach very gently.
> Might stick in your craw, but will have a slightly better chance of success.
What I 've found works is to approach openly and enthusiastically friendly, with no hint of any 'issue' to be discussed. After a bit of a chat you can generally inveigle enough info about them to have a good idea of their identity. At this point you can can politely address the real issue. Since (a) you have established an initial rapport, and (b) they realise that you 'know where they live', they tend to be more amenable to behaviour change. And if they don't comply then you know where to go next to sort out issues.
I 've used this approach successfully 3 times in the last year at one of my local crags where (a related) set of organisations has regularly had groups causing issues with local climbers. Most recent occasion was when one of their groups taped off one the main sectors to discourage use by others (and in so doing discouraged you from accessing other sectors). They then left these tapes in place over the weekend (because they were returning the next week to continue)!