Consent may be withdrawn at any time before or during an activity. If consent is withdrawn, all activity must stop immediately.

Consent may not be withdrawn after the activity has ended.

If you are not comfortable with something and you make that clear, a responsible, ethical player will stop immediately. But you cannot revoke consent after something has already happened that you had given consent to.  You may feel regret, but that is not the same thing as you having your consent violated.