Using a lock switch as recommended above is good (computer cases with doors used to come with those standard so when you were away from your machine you could disable the power button and lock the door (if the case had a door) at the same time - the reset switch would also need to be disconnected or hidden behind a door. Here is a page of replacement keys and key switches much cheper than those mentioned in another post:
http://www.allelectronics.com/...75/Switches/Key/1.html. Can be used to disable the power or reset buttons. Should be some out there with panel/door lock as well.
Here is a case that is a lot like the Silverstone Berserker I had at one time. That version was very secure - it had a way to lock the front bezel in place and the right side panel. The left panel had a panel lock loop, so you could make it so someone would have to be willing to break the thing to get in.
http://www.servercase.com/miva...ode=Entry-Level+Server
the other two versions have 3 more 5" drive bays in the bottom, so you'd have to get a drive cage to put HDDs there, but there is room for 1 or 2 HDDs below the floppy bays and above the bottom 5" bays.
This one is even closer to the Berserker in terms of security and looks much better:
http://www.buildtoorderservers...egory=55&idproduct=478 . But unlike the others, this one still has 80mm fan mounts in the rear. All are variations on basically the same chassis.
Used to be a company with Arrow in the name that also sold the CK800A,B,C at better prices than the above, but I can't locate it now. The B and C models are in the "Domino" series for an obvious reason.
And the Chenbro workstation/entry level server cases are reasonably well secured. The SR105xx series has a longer chassis to accept extendedATX (EATX) mobos as well as std. ATX, while the SR209xx is shorter and takes only standard ATX mobos (put 68 where the xx is for my fave version - I've never seen other than the 68 or 69 version here in the States). The stock fans can make your ears bleed at full speed. I put one of my C-M fan kits in the rear and a Zalman FM2 92mm in the front with something like a FanMate 2 on it. Some of the Ark pedestal server cases (avail. on Newegg under Cases/Server/Pedestal) are like the Domino series. Looks like the Egg dropped Chenbro's pedestal cases - they used to have a good selection of 'em. I think SuperBiiz.com (yes, the double "i" is correct) still carries them at decent prices - prices went up about 20% but they're still there. Check the Chenbro.com site for detailed info and more pix. Good luck!
.bh.
PS: The Antec Titan series may also be something to check out - Similar in concept to the SR10568 but bigger and pricier, but I think still in your range. .bh.
PPS: Possible to lock the room the confuser is in - not easy for ankle muncher to play with buttons then?