Forum spam tends to happen between 3am and 6am eastern from what I can see when few to no admins are online.
I propose you do not hire more admins who are awake at this time.
I propose you hire a "moderator" or two.
Specifically my proposal is as follows
Create, within the hidden admin forum, a subforum, called "trash"
Create a new usergroup called "Moderators".
Moderators would have only one power that players don't have. The ability to move topics.
Moderators would be told that if they ever move a topic to a forum that is not the "trash" forum, that they'll fired. Well. Maybe not that harsh, but you get what I'm saying; moderators are for one purpose and one purpose alone, moving topics to the trash forum. I'm sure if they move a topic someone honestly posted in the wrong place to the right place it won't be seen as come kind of coup d'etat
Moderators would not be able to see the admin forum. They'd know it exists, and what name it has because that would appear in the menu when they choose where to move things, but they wouldn't be able to enter the admin forum.
When a moderator finds forum spam, their duty is to move it to the trash forum.
An admin can delete it later.
I also nominate edventure and grumpchkin as moderators as they are 2 of the 3 people who most often find forum spam and the 3rd is not a mentor.