I'd think that you've done about the best you can do in this situation.
As to;
Hired Goons - I'm sure it's just as easy for the perv/perp to hire them against you and yours, so I wouldn't recommend that course of action. If he IS juiced in, or that crazy, you might just come to work at a smoking pile of ashes in the morning.
Police - From what I've heard here and from others, I wouldn't count on the Thai police. I've been told by a US cop that threats are not a crime so I doubt Thai police would act on them.
Inform Agencies - That seems a no brainer. Haven't you all ready done this?
Undercover Ninja - Sending someone to get groped who can whip a quick TPR lesson on this guy sounds amusing, but do you feel it's your place to escalate the situation to violence?
Let a Thai handle it - This seems like the winner to me. If the system of patronage is what I've read here, that's your only reasonable action. You don't want to walk into a lion's den wearing a porkchop vest, right?
My own addendum as to the harrassing messages; I hope that you are not answering them. The best course is basically to ignore them. My cell phone and email have blockers on them so I can choose not to recieve communication from people that I don't want to talk to. Does your and your friends'? Use it!