I genuinely thought that I would be clicking through to an insightful post about how many crappy web-based "business productivity" apps that try and go the 37signals/Fogcreek route to success, and why the glut in this market helps no-one. Shame.
We never implemented that feature. CEPS provided usage logs, and if a user felt that someone was hogging or abusing a printer, he could simply talk to the abuser.
That's much more likely to have a successful outcome for the company than putting up a technological barbed wire fence across someone's path.