According to halocho one is not allowed to be in close contact with his ex wife or ex husband as this can lead to lewdness even if they do not get remarried, and much more so if the women remarried or when the husband is a Cohen for in these two cases there is a Torah prohibition for her to remarry her ex husband.
For this reason there are restriction on how close they are allowed to live one to the other for example they should not live in the same apartment building .
Shulchan aruch rules they may not come together for a court case as they are not to be in direct contact if they do they are to be punished.
If they have demands from one another example one owes the other money or one wishes to send to the other food and the like this should all be done via a messenger (shliach) but not directly.
When there is important reason for them to talk example regarding the children’s needs then it is allowed it should only be done over the phone for this purpose and they should not talking about anything else if there is need to meet face to face this can only be done as a last resort and there is to be someone else present in the room (in addition to the fact that there should be no concern of the laws of yichud).
So therefore, one should cut contact with their ex wife.
According to chassidus and kabbalah a couple which marries create a unity of two halves of one soul when the couple divorces these two halve could not unite any more at the source as they where not meant for each other or they’re unity was only meant to be for a certain amount of time.
