Nip it in the bud.

Address a problem, harmful tendency, or unwanted development while it is still small. Early action is presented as the best way to prevent it from becoming harder to manage.

The image is of removing a flower bud before it can open, so anticipated growth never occurs. It can imply decisive prevention and a disapproving judgment of what is emerging, making it potentially harsh when directed at a person, idea, or aspiration.

Use it to urge prompt intervention when an issue appears likely to worsen or spread. It suits discussions of misconduct, conflict, risks, or bad habits, but may sound harsh or dismissive when applied to people, developing ideas, or aspirations.