the best way to make a clean break is to decide, for yourself, that it's what you want. Start as you mean to go on. Don't text him anything.
If you send a msg saying I will never speak to you again (or something to that effect) what you are really doing is inviting him to respond. It's only half-hearted, because really there is the secret hope he will suddenly had a 'see the light' moment and beg you not to go... but it never works out like that :( what is most likely to happen is he will respond with more abuse, and wear you down a little more.
If you want a clean break, even just for a little while, the best way to get one is simply to stop responding. I did this in my 8th month, I just refused to open texts, sent emails to a separate folder that I ignored and basically switched off from the ex. I didn't think I'd be able to, but it felt amazing to get back some control.
We have two children so I've always known he would be around every week and I'd have to deal with him when baby arrived, but even a month of no contact recharged my batteries a huge amount. If you want to feel better, I strongly recommend you give no contact a try.
FWIW my dad has been so angry with my ex it's made him literally sick, and he's not always known how to deal with it. your dad probably doesn't know what to say because he just wants to punch the knobhead out, and doesn't want to upset you by saying that! He will come round, he loves you.