Preventing users from office A viewing reports from office B or vice versa

Preventing users from office A viewing reports from office B or vice versa

I have only 1 form for data entry that is accessible by office A and office B. One of the fields is the Office Site where users select from the dropdown either office A or office B. I would like to prevent users from office A seeing data entered by users from office B. How do i achiveve that?