I'm not sure. "De una vez por todas" sounds more like a settling of scores, but this expression is an expression of agreement. I couldn't find where it comes from, but maybe something like "ser de una misma opinión".
I found
this place, where it's defined as Argentinian slang, and I guess 2.2. may be understood as "de una (sola) vez" (in Mexico we might say "de un jalón").
