As Gary says, theres not a lot you can do about it, im afraid.
To go onto the beach and confront people isnt usually an option if you are out with the family, or on your own, which is usually when you witness these things!
The phone number of the fisheries officer for north wales ( Dee estuary to Bangor ) Phil is, 07920134249
please try to avoid calling after 2100hrs as his wife will give him a hard time.
If you find anything suspicious, net wise, without names or identification on it,give him a ring.