I am looking for a cabinet to store a full sized tower PC in. My
problem is I have a web server that housekeeping keeps turning off. I
have even gone as far as unplugging the power button so they can't
push it. Now they just unplug the power cord from the back. I want to
know where I can purchase a cabinet that holds just the PC itself. The
only ones I have been able to find are the ones that hold the PC,
moniter, and printer (and cost $300).

Ikea sells some cabinet, you just need to put in a power bar and drill
some holes for ventilation.
If you budget is more you could get a nice switch cabinet with fans in it.
If you actually by a UPS and tell the housekeeper that it is beeps,
undo when she has done or it will Explode.

My budget is $100-150. I have a UPS and they just unplug the power
cord from the back of the computer or from the UPS itself.

I think this is under the wrong classification, it should be under
Human Resources....

I don't get it, who PAYS this "housekeeping staff" ??
Why is this person/people still employeed there? 
Do they not speak English?  Who is in charge?
Have you tried watching the person and when they get close to
unplugging --STOPPING THEM?

Also, I don't see how a cabinet will help, what's stopping someone
from moving the cabinet out from the wall and unplugging the Power
Bar?

Sign me confused,
~~Cynthia

Number 1, the housekeeping staff comes in at around 10:00 PM. If you
want to volunteer to come and wait for someone to come around, be my
guest.
Number 2, A cabinet will help the problem because if they unplug the
cord from the wall, the UPS (which will also be in the locked cabinet
if I can find one) will start screeching.

ahhhh, That makes sense.  I still don't get why the staff won't follow
instructions, but hey, it's not my business, in more way than one.

why not build one your self from things you can get at home depot?
cost not very much for some wood nails and a latch with a lock. cover
it with some thing and put "high voltage Donot touch" signs on the
stupid thing. total cost to you $50 and 2 hours of work.

You don't need a full enclosure:

You can use one or more of these suggestions:
1- Build a small box around the power supply
2- Superglue the power cord on both sides
3- They probably want to use the outlet for a VACUUM CLEANER, you can
get an electrician to run a power cable to your pc from inside the
outlet, and leaving the outlet available for them to use.
4- A Huge/Colorful Sign/Sticker near both sides of the power cord (in
multiple languages if needed): WARNING: DO NOT REMOVE POWER CORD OR
YOU WILL BE FIRED
5- Write a memo to HR saying that you have recently lost important
data worth more than $1,000,000 and that you hold them responsible.
6- Attribute any problems you may have on this server to this issue.

A viable option is to screw the power strip plug into the wall. 
Simply remove the screw from the socket cover and then re-insert the
screw through the metal ring of the plug and the cover.  If there is
no metal ring for the screw, one can be easily attached.

Housekeeping would not go to the trouble of removing a screw just to
turn off the computer.