You only have to say the boundary once. You have to back it up every single time.

Stating a limit is the easy part and it is not what makes the limit real. What establishes it is the behaviour afterward, since people learn from what actually happens rather than from what was announced, and a rule enforced occasionally teaches that it can be waited out. Repeating the sentence instead of acting on it makes the words cheaper each time.

Decide what you will do rather than what you will say the next time it is crossed. Then do exactly that, calmly and without a new speech. Consistency is what turns a sentence into a fact.