That's why some restrictions would definitely be needed, e.g.:
- 1000 earned Merits
- 1000 activity
- positive trust score (or only DT1 members?)
- the "button" has to be pressed 5 times
- maybe a "counter-button", if the "wtf-button" was pressed falsely
- 1000 earned Merits
- 1000 activity
- positive trust score (or only DT1 members?)
- the "button" has to be pressed 5 times
- maybe a "counter-button", if the "wtf-button" was pressed falsely
Perhaps instead of these requirements, the demerit function could be restricted to moderators of the board, requiring only one press of a button. Just like how the reporting system can be used by anyone but only mods can actually delete posts. Newbies would start complaining about bias or "keeping them down" if legendaries were allowed to do this.
Make it cost 2 smerit to remove 1, and don't allow merit sources to participate. 
This reminds me of the way Stack Overflow implements downvotes that remove reputation. That's another possibility. But it would have to be restricted to people with say 1000 earned merit, and staff I guess.