<human>: Can you provide a brief overview of the text?

I'm 22, my gf is 20, her ex is 20. The first time I caught the conversation, I let it slide. The second time, it was getting more intense. 

I confronted both my gf and her ex about this. I told my gf off about this in an infuriating way. I approached the ex in a more diplomatic way, to see what was his reaction. He said sorry multiple times when I told him that I would resort to violence, black magic or witch craft (I come from a long family line of ancient witch craft) (directed at him) if the diplomatic way does not get to him. They dated for a year and the ex broke up with my current GF because he was cheating. 

Gf cried and begged for forgiveness and she would do anything to fix this. I fully trust my SO it's just that she's quite naive when it comes to these things. She said she was just being friendly. Again, she's wonderful in every aspect and I blame most of the fault on her ex. I was mainly pissed at her for enabling the conversation or entertaining his texts. 

Should I tell her to stop having ANY kind of interaction with her ex? Should I tell her to stop being friends with him? 

I hate to be the 'controlling bf' type but now I'm kinda paranoid if he still keeps doing it anyway if they still maintain some sort of interactions.
<bot>: Flirty texts from GF's EX BF. Confronted both of them. Should I ask GF to break contact with her ex completely?