The fish poachers etc (the red exclamation mark) are part of the narrative in the game pack, so no bug. There is an interaction on them to ask them to leave nicely or rudely. Sims often stop what they are doing if another sim starts interacting with them, so it is perhaps possible to stop them that way, The island clean state needs to be maintained, that is, to keep cleaning up and telling sims off etc. Carl's guide is pinned to this page.
Oh, and also: the fishing sign at the mentioned lot is in the world area, not the private residential lot. If you want private, place fish traps on a lot instead.
The fishing spot at Admiral's Wreckage at Mua Pelam is where I mostly see the fish poachers. What happens if you place the fish traps somewhere else, and not directly by a fishing sign?