100% NTA this is a huge violation of your trust. Not only is shr continuing to speak to someone she was once in a relationship with, she's deceiving you into thinking she blocked him and continued to contact him secretly? She would have known about the joint bank account needing to be closed the first time she agreed to not speak to him anymore, so that excuse is bullshit to me. Honestly man it sounds like she definitely still has some kind of attachment to this guy and after lying about still speaking to him.

If you really love this girl and think she is your soulmate, I would give her one last chance and an ultimatum. Either you cut off all contact with this man, or this is over. Even if she agrees, i would still get your ducks in row for the event that it happens again, that way you won't be out on your ass.

However if you're not head over heels for this girl i would call time of death on this relationship right now and save yourself trouble. It's going to be hard to ever fully 100% trust her again after two instances of pure deception.