If blocking and deleting is the only way that allows you to get over someone then go ahead and as Atom said you dont own any explanation to anyone, let alone some girl which decreased your quality of life.
You dont give explanation, you make your intention clear unconditionally, its not a trading process nor a negotiation.
However deleting someone could create reactions like this one which could potentially sabotage your attempt, after all now you're thinking about her while we discuss this, so you just simply leave things how they are and go no contact.
Just disable any notification coming from that person and only after few weeks or a couple of months, proceed with the elimination.
You avoid the drama and the satisfaction that such person may feel, in fact when somebody gets deleted right after rejecting someone feels so important to control other people reaction through their behaviour.
Anyway if you feel you cant keep such person without the risk of contacting her again then yeah, delete as soon as possible.