I had a similiar situation yesterday, at a fairly well known fishery.
Had a good morning session catching some nice carp, when 2 lads and a young girl turned up, with the bare minimum of gear including no landing net. They then decided they were above the fishery rules, and started "stalking" the carp and using floating baits wherever they fancied, both against the rules.
When I had the luck to hook into a decent carp (and personal best I might add, being my first double figure at 10lb 8oz) they all crowded round making playing the fish even harder while constantly playing "rave" music through a mobile phone. Luckily this place has 8 pools so my brother and myself could move to another lake well away from them but not everyone is that lucky.
The worst part of all this for me was the fact the bailliff was happy to take their money without saying a word to them about their behaviour even though a few other anglers had specifically spoke to him about them.