Yes, your ex-employer can legally ban you from visiting your former work permises. The fact is that absent some form of legally actionable discrimination or retaliation, an employer can institute a ban on certain former employees from entering onto a private (or sometimes even public) premises. To violate such a restriction can result in the criminal charge of tresspass. Accordingly, unless you are afforded protection against this under the terms of an employment agreement or union contract, you cannot re-enter your former place of business (at least not without your ex-employer's permission).