Saw about 10 teenagers on the 18th last Sat, kicking a football and going in the lake.
Although fuming, went to security at the front gate instead of confronting them myself and after a bit of trouble on my part to get them to understand, they despatched a security car to them and they were off within in 10 mins. Well done to them.
So moral of the story is, if you see anyone on the course doing what they shouldn't, tell the security and they will sort it out.