Disable iCloud in Lab?

In one of my computer labs, I have some iMacs. Someone took it upon themselves to use the iCloud feature and now that machine is locked down with a 4 digit pin. I was wondering how I can protect the remaining iMacs in this labs to prevent the same problem from occurring again. Is there a way to remove this feature completely? This is a computer lab and not employee macs and we don't mind them using it, but lesson learned that I don't want students being able to do this.

Any suggestions or idea?