I have recently taken up a new post as course leader but face the dilemma of having to manage someone who internally applied for the same post. This individual has already demonstrated a complete lack of regard for my leadership. I have let it go so far, thinking that they would grow up and get on with it but this is clearly not going to be the case. I do not want matters to spiral out of control and involve disciplinary action.
You missed a good opportunity the minute heshe first stepped out of line.
By letting it pass you gave this person the advantage and showed yourself to have a weakness for confrontation. An effective manager must be prepared to confront issues, however unpleasant. Deal with it now. Calmly censure them giving them your reasons for doing so. Keep a record of all meetings.
In the last resort take disciplinary action.