My suggestions are through following questions:
a) What are the office hours?
b) Why visitors visit your office? How many visits are there per day? Have you done analysis of their visits? Can you create separate help desk for the subject on which maximum queries originate?
c) What you can do to prevent their visits? Can you be proactive in providing information through your website? Are visitors required to obtain some form, fill the form and submit the form in person? If yes, then can all this be done through website?
d) Can you be proactive in providing the information through e-mail, SMS etc?
e) Can you fix visiting hours for the visitors?