One of the hardest decisions in life is to be ethical, especially when those around you are not.
Depending on exactly what type of people these are, you can base your judgement of whether or not you can reason with them. It sounds as if it might not be a choice.
The law is supposed to protect law abiding citizens from those who who are not, we all know how well this works. I wouldn't visiblely lead a neighborhood crusade against those who will take retribution at an unknown time, while having tons of witnesses who say they were somewhere else etc...
So, have you complained yet? If not, do so anonimously, see if something happens.
Animal issues: I have two friends who live in the same very small medieval village, a composer who has a very docile dog and an artist who has a rambunctious scoundrel chicken eater and trespasser, however, it's a very nice dog.
The artist's dog one day growls the composer's dog into a corner and the composer calls the animal squad and they take away the dog and put it to sleep. The artist, very civilised man, swears that he will never be friends with the composer again, even though they live 150 feet from each other.
These are 2 educated rational men, the dog that was destroyed may have been truly dangerous but its owner couldn't see that through his love for the dog. Now these 2 don't speak to each other, before this happened they had a good friendly eat dinner at each others house walk in the woods with the dogs and the kids relationship. Feud.
If these two spoiled a relationship over a dog, think about someone you don't even want to know, who's probably not such a nice guy in the first place, might want to do. I wouldn't even let them know that there was a studio in the neighborhood, are they thieves? etc... You'll need the SAS's help before you're through.
Your right is to complain and remain anonymous as long as there is no crime involved. Otherwise I would sus it out well before I took any other kind of action.
More soundproofing?