Unfortunately, this isn't a feature available, but it's definitely one that's a top priority.
:-/ I'd like you to reconsider this.
I refuse to ever delete or modify anything that another person posts, even if I disagree with it.
I think it falls in line with the "I never want to ever delete anything ever" idea. If locking cannot resolve the thread issue I'd prefer to have the user censor themselves... If it is still an issue then you can reconsider their place in the guild.
I just think it's asking for trouble to have moderators edit forum posts.
--