This is a 4th amendment question. Does anyone know and have a good legal answer?

[John S]

I went to a public (not private) beach in the US last summer. Alcohol is not allowed on the beach, so the police (not private security guards) were guarding the entrance to the beach. They were asking to search people's bags and not allowing those who refused a search to enter the beach. This does seem like 4th amendment violation to me. My question is do the police have the authority to refuse entry of citizens to a public beach because that person will not consent to a search? Would this answer be different if a private security guard was guarding a public beach? If you need more details please ask. Thank you.