Hilly, I want to understand your view of "trashing". Do you mean messing up the area with trash, or ripping your fine dock apart for the local's own use?
The environment at the river probably cannot be compromised much more than it already has been. Now if you talking about disassembling your dock and taking it home, barbed wire will not be a deterrent. In a previous life, I used (in USA) barbed wire to keep unwanted visitors from many different types of areas, with only a challenge to be more prolific and inflict much damage.
I do think that corvettelover's general ideas, with a rollered ramp, are appropriate for Thailand and materials available. I did find this a somewhat interesting Site, for the basics. Instead of pipe anchors, I would consider using treated timber pylons, buried deep enough to eliminate a washout. I'd vie for something like this.