Boundary Violations

Can you set a boundary after the fact (in the format, “Because they have violated this boundary, I will do Y because X has occurred”)?
The model for BV seems to be, “If X happens again, I will do Y.” Does it matter?

Also, I think Brooke said in the seminar on Boundary Violations that you don’t need to tell the person what will happen if they do X, and doing so would come across as a threat. Am I interpreting this correctly? It is something you have in your mind that you will do, but you do not (necessarily) tell the person?