My listing agent's policy was: *Every buyer must be accompanied by an agent.*
If they didn't have one, she would give them the contact info for one who could help them. That took care of that. Unattended minors, grown-ups with bad intentions, buyers who are not serious about buying, and lookieloo neighbors need not attend. As a seller, it worked out perfectly fine for us. Thankfully, she was not an agent who exploits their sellers' open houses as a meet & greet to drum up business for herself.