So if the ex of your partner is starting to reach out to him/her, how would you react?

I do nothing, at first. If she acts in a way unbecoming of the values our relationship is based on, I leave, no questions asked nor needed.

People aren't (usually) stupid or naive enough to not know their actions have certain consequences. If she is that stupid/naive, we are definitely not meant to be together since I abhor those traits.