grammaticality - “Don’t say” vs. “let’s don’t talk about it” vs. “let’s not talk about it”

Imagine there are three individuals in a small room. Person A is going to say something that person B should not be made aware of. What sentence should person C use to stop A from continuing his speech, such that B wouldn't notice that C does not want him to know?

Here is a possible conversation:

A: I went to the cinema and met...
B: Oh really, you went to the cinema? And met whom?
C: Hey, don't say!
A: I cannot remember anyway.

Is this use of “don’t say” natural and idiomatic? What about the following alternatives?

• Let’s don’t talk about it.


• Let’s not talk about it.