The Police will assist in two ways if you can get them to the venue.
1.Assist you removing the individuals.
2. Obtain names and addresses for a)prosecution by the EA for fishing out of season but only on rivers and b)rod licence infringements.
The difficulty will lie in my first sentence.
The advice you will probably get is to do 1&2 yourself.
Fish theft is another matter of course so you might want to drop this into your conversation with the Police.Additionally a ticket or two to angling members of your local constabulary works wonders. Simply pop into the local station with an advert for members.