no one can enter your property without prior permission. As far a entering without a broker, there is no legal reason why they can't, however any negotiations should be conducted with the brokers present
Simply put NO. A buyer can not enter a home unless they have permission and are accompanied by their agent. The only if would be if the seller gave them perission. Where is the listing agent in this situation. They need to take control of the situation and stop it., Start with changing the code on your lockbox.