Hide User Posts
Selecting this option from the Hotspot down-arrow menu adds the user to a personal list whose posts will all be collapsed from your view by default. This option works similarly to when you hide an individual forum post for Personal reasons, with the exception that BGG automatically hides these posts before you ever see them. On some forums the list of users you've hidden is officially known as a "ignore list", "blacklist", "black list", etc. This option is casually called "plonking". If you plonk someone, you won't see posts by them, but of course could still see their text quoted in other peoples' forum posts.
Note that talking about who is in your hide list in the forums is a zero-tolerance offense and will result in immediate moderation. Moderators will not discriminate based on apparent attempts at humor. Aldie made a statement about that here: http://www.boardgamegeek.com/thread/255246
There is currently not a way to avoid receiving geekmail from a user. More discussion of that was here: http://www.boardgamegeek.com/thread/304100/