What kind of proofs or documents I should submit to court for me to stop paying my ex's alimony?

If you are careful and savvy, you may be able to get out from under the burden of alimony. But you will only likely get one shot at doing this right. In Utah, you cannot terminate alimony unless you prove that your ex-wife in a situation like this is cohabiting. That does not mean she merely spends a lot of time overnight at a boyfriends house or that her boyfriend spends a lot of time overnight at hers. "Cohabitation" means that an couple lives together like a married couple, but is unmarried. The idea makes sense when you think about it; if someone is going to "shack up" with another without remarrying for the purpose of trying to keep alimony going, then the alimony should terminate. You may have to invest some money laying the groundwork for proving cohabitation. If your ex-wife is cohabiting the things that you're not aware of it, it may be some of the best money you ever spent by hiring a private investigator to gather evidence proving she's cohabitating before you file anything in court. Terminating alimony requires a great deal of evidence to persuade the court because terminating alimony is a major change that courts are reluctant to grant.

You need to file a motion to stop maintenance.

Well, you could possibly submit a petition to modify the alimony based on a change in her circumstances. However it sounds like you just have rumor and hearsay which would not sway the court. You need some documentation that her bills are being paid by some other person. You might be able to get this in discovery, but it would be very difficult. Speak with your attorney.

You have the burden of proof, and an PI would be helpful, as would her mailing address and testimony.