If the user is doing trade under that name in the same business you are in (and/or leading people to believe they are you) on the same forum, then yes you potentially could build a credible case against the person / company behind that user name.
Though, it often may be easier and less expensive to present your case, in a matter of speaking, to the forum operator and strongly request they reassign / block that user name due to TM issues / passing off. Most forum operators will do what they reasonably can to avoid potential legal problems, including reassigning (especially inactive for long time) / blocking names, deleting posts, removing links, etc.
Keep in mind though this goes both ways - they could fight back and demand that same username on other forums where it's already been assigned to you. Could get nasty, especially if they have a strong TM claim and/or are well known under that username by others - they may gang up to retalate with negative posts, DDOS against your website, etc.
With all that said, if the username has been long inactive on the other forums - contact the webmaster and see if they're willing to reassign it to you or at minimum suspend the name. If the username is active, try contacting the user - perhaps they'll be willing to change it / stop using it in exchange for something, such as free stuff, money, etc.
Ron
