If you are an American citizen, can they prevent you from entering the country? I understand they can delay you, but I don't think it can be indefinite.

^ This right here.

They can deny foreigners, but I've always read that they cannot deny Americans in unless their citizenship gets revoked, I guess.

Not even that. Your right to enter the country, as a US citizen, is "Absolute, Unconditional, and Irrevocable."