just because you are in the right doesn't mean that Monster is exactly in the wrong: COPYRIGHT LAW is in the wrong. Because it is stupid. I am far from an expert, but here's the basics.
If some new company comes along and calls themselves, "Monsters" energy drink, puts it in a black can with a big green M on it and starts raking in dough, Monster can sue. The Monsters people can then pull out your forum logo and say, "Hey, you didn't sue THESE guys. Clearly your claim to Monster as a brand is forfeit."
It probably wouldn't work, were Monster to pursue a lawsuit because my example is extraordinarily flagrant, but it would be admissible against them.
The lawyers probably don't actually care if they win. They have no interest in the forum, or any money in damages they might get. They just want to deny ammunition that might be used against them in a future copyright battle. But because they are a multi-million dollar company, even though they just want to put up a fight so they can keep their trademark, any fight whatsoever is probably more then a small group of hobbyists can fight against. Of course, since they are so huge, they also don't really care if you get crushed along the way. This isn't David versus Goliath, this is just a company acting out of self interest and the crushing of people is purely incidental.
Doesn't make your situation any less sucky, and good luck. I'd look up your local SLAPP laws, but honestly, I'm not even sure those apply to this sort of thing. Failing that, there might be some sketchy, uncomfortable legalese filled workaround that still lets you keep your site name, and lets them keep there ability to defend there brand name intact. It may be annoying but hey, the enemy is dumb laws, not Monster proper.