So my parents have a property that has a creek running through it next to a public reserve which they love, however almost every day there seems to be people climbing up the creek, past their fences on the rocks or in the water to look at and/or kill the native eels.

This in itself becomes a tricky problem when people climb up the creek onto my parents property (sometimes at night), inviting themselves to have a look around their garden and sometimes even look into their windows! They have a big "NO TRESPASSING - PRIVATE PROPERTY" sign, but this seems to do absolutely nothing.

I believe the legality around it is that they own the land under and around the creek, just not the waterway itself.

Does anybody have any tips on what to do to keep these little shites out? I have found my general yelling approach just seems to invite arguments.