As it isn't possible, this isn't really relevant, but I thought I should clarify anyway - my thinking was more along the lines of a new category of forum user, trusted members chosen from the active userbase given an upgrade and not any Tom, Dick, or Harry newbie.
As they'd only be able to hide the spam and not actually delete the content (or even ban offending posters) as that would fall to a full moderator to do, it should help prevent any abuses of power - something that recently happened to another forum a couple of months back - and if any of the spam-hiders were found to be playing games they could easily have their privileges revoked.
But as the forum software doesn't have the capacity for that level of customisation it's a moot point.