The community I ran decided to do that one day, forum raid some innocent forum. I got a letter from the data centre I host at threatening to close the account lol. It's not a good idea!
I've been on many forums that have done invasions before - there is nothing legally wrong with a group of people signing up to a forum and making posts as that is what they are there for! The DC in question was blustering and would have been in breach of contract to shut down the account, you should have pointed this out to the them.