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Frankly, a determined hacker with know how is going to continue to do this whatever you do. My advice? Either report your roommate to the appropriate authorities or get a new one.
 
My roommate keeps creating his own administrator account on my mac using this method:

http://osxdaily.com/2010/08/10/forgot-mac-password-how-to-reset-mac-password/

I tried changing the permissions on the .applesetupdone file. Short of locking away my computer all of the time, any ideas on how to protect from this?! I can't believe it is this easy to get past password protection.

Thanks!

Try Filevault2.

If your roommate can get around that please do let us know.
 
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