Post a short, general list of rules in several obvious places. Make them reasonable enough to cover most possible user needs but flexible enough to cover things that you haven't thought of yet. Any user who is stupid enough to break the rules by running fork bombs, torrents, mining, hiding stashes of lemur porn or anything else which a child of six could tell you was a bad idea, will have their accounts disabled as soon as they are discovered.
If they have a good excuse for abusing the systems then discuss it with them, suggest alternatives to running rendering jobs on the lab servers and keeping passwords on sticky notes or whatever else it is that they are doing wrong and then restore their access, trusting that they will know better. If you do it right, they may even decide that it is better to ask for permission than forgiveness next time.
If they don't, send a memo to their department head briefly outlining what they did, how it was detected, what action you have taken, and that you won't be reversing this decision until you see a presidential pardon come down from an appropriately high authority. It doesn't matter if they have Really Important Work which needs to be done by the end of the week or not, just cut them off until the proper User Apology and Restoration procedure has been completed.
There you go. This solution is licensed under the WTFPL which is compatible with the Open Source Definition and the Debian Free Software Guidelines so you can use it any way you want. You can even supply your own LART and display it prominently by the door of your office if that helps get the message across.