@ismail123,
Hire a 24 hour a day doorman/receptionist. You HAVE to pay a livable salary because if you don't you'll likely end up hiring the type of employee who will end up stealing from his or her employer (you). So, pay a livable (decent) salary with benefits and you can be picky about who you hire.
Just remember to do a criminal check on your new potential hire BEFORE officially hiring them so you can weed out the crazies, the thieves, serial killers, etc....