I'm uneasy about a blacklist, especially forum-wide. A PM is private, it's meant to be exclusive just to get something done or goof off with the specific people involved. It's minimal. Forums are supposed to be public discourse where people can participate so long as they follow basic rules.
**There's douchebags that can hurt or ruin a thread. But there's also OP'ers (and GMs) that are douchebags that can hurt or ruin a thread. Giving someone the ability to ban anyone on their own whims is a bad idea. Even those well-intentioned will arguably screw up on that end, where they ban the slightest conflict, or ban something in favor of their own opinions (e.g. someone on one side of the argument between people that were upset about/complained/criticized RPG's long downtime and wipe and people who didn't like the former group's actions).
It's an issue of censorship and elitism (almost a trigger word with the donations talk, but it fits). Members could completely block out others just because they rub them the wrong way, or because those people don't agree with them. It could establish more official cliques than just the same people tending to hang in the same circles around the forum, which leads to some superiority complexes and categorization.
Why the report button works, under a good staff, is that it spots someone maybe breaking the basic rules of conduct, and goes through a filter of someone who is less biased, knows the rules better, and whose goal it is to keep those rules in check enough to make things fair and civil.
***Just get the report button back. RPG is a good place. The staff is solid, they don't screw people over because of favorites or pettiness, and most of the members here are decent and not that bad. Even the douchebags are usually mild douchebags. Very few hang around (a) thread(s) just to personally attack and harass other people.