Yes, a private business (be it your employer or a merchant) you choose to do business with may elect to not allow firearms, even when licensed, to not be allowed on their property. An employer may choose to terminate your employment if found to be carrying and as a customer a business may ask you to leave. In some states a posted "no firearms allowed" sign carries the weight of the law and you're "guilty" by ignoring it and continuing to enter the establishment. In thus case if you are reported because they see your gun for whatever reason, you can be charged accordingly to that law. In a place where the sign carries no weight, all they can do is ask you to leave or remove your weapon before returning. If you fail to comply, then they can try to cite you for trespass and get law enforcement involved.