Pretty much you would have to delete the post on here and then wait it out till the next time google indexes it. I'm not sure how long you have on here to edit/delete something, so if you can't you'd have to ask a moderator.
Remember with the net, once it's been posted once, you can never for sure "get rid of it", someone could have a copy out there even if you delete the original.
It is a good sign for the forum though, more popularity, more you can charge for advertising. I'm on a forum for limousines, and they are indexed a few times a day and hit high is results. They made a specific section that is only accessible if you are logged in so it will not get indexed.
Also, depending on the forum, can be good for you site if you are linked to it in your signature. (hint. don't link you company name, link keyword/phrase to the site, google weighs what the actual link text is)
Of course, as I said, it depends on the forum setup. Another forum which I am a moderator on gets so many spam signups to get the links in the signature, the decision was made that all signature links are "No-Follow" (tells the bots to completely ignore them) and signatures only show when body of post is past a certain number of characters and you have to have 10 qualifying posts before it will display at all. (It lead to spammers just doing like 10-15 posts at a time, but sure makes that easier to spot
This is also why I mentioned in the one thread where someone was asking about his quote of a property about concern of people finding it. The client possibly find it if they google the company name. (this on top of competition knowing what to bid to underbid you).