To turn off your signature for a specific post, check the "Do not include my signature in this post" box at the bottom of the posting page/window.
If you would like to turn off your signature for a specific forum, click the "by default turn OFF my signature in this forum -- it is currently ON" link at the bottom of the posting page/window. You will still be able to override an individual post's settings - this simply changes your default setting.
To turn off all signatures in a forum, please see this wiki article.