My office is near a computer lab which is frequently closed. When it is closed, students come inside my office and demand to use my computers, my phone or worse, demand that I open the lab so that they can use it. I do not have access, and I have put signs on my door , but they still keep bothering me. Charge them money for the phone calls. Alternatively, you could install a trap door in front of your desk.
Pepper spray is always fun, too.
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