Any property owner (or tenant if the property is leased) may ban any person they want from their property, with 1 exception. A place of public accommodation (like a restaurant) cannot ban people fromthe premises due to some form of legally actionable discrimination (i.e. based on their race, religion, gender, disability, age (40 or over) or nationality). Accordingly, you can be banned from the premises of your former employer for any reason or no reason at all.