Disable a specific user's ability to give rep to others

[DonosOdD]

Here's the situaysh:

A member of my forums was giving negative rep left and right as he pleased, and disregarded the staff's advices. He was banned for that but I want to avoid banning from now on, so I figured I'd disable his rep, right?

Wrong. His rep is disabled in the admincp (I don't allow users to disable it themselves), but he still can give and take rep points. How do I disable that without disabling it for everyone else?

Thanks in advance.