once and for all
once and for all {adv. phr.}
1. One time and never again; without any doubt; surely; certainly; definitely.
Let me say, for once and for all, you may not go to the party Saturday.
For once and for all, I will not go swimming with you.
2. Permanently.
Bill and Tom asked the teacher to settle the argument once and for all.
The general decided that two bombs would destroy the enemy and end the war once and for all.
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Source: A Dictionary of American Idioms