An employer is not required to intervene if one employee is stalking or harassing another. It is unwise for them to not do so, if the complaint is credible, since if you harmed, etc. by him, because they were aware of the risk or threat, they can be then be held liable for your injuries; that is, while  they cannot be prospectively required to act, they can be held liable after the fact. But they can choose to not act and take the chance of future liability.If the guard's behavior crosses the line into criminal stalking or harassment, you could file a police report against him, possibly press charges, and possibly get a protective or restraining order. You wish to go to the local police station and discuss the situation with a detective, to explore your criminal justice options.