If a spouse got a divorce for irretrievably and was having sex with the other spouse and it can be proven what can that broken spouse do?

In Nevada, either spouse can obtain a divorce by claiming irreconcilable differences. A court would still grant the divorce, even if the parties continued to have sexual relations because the views of the parties could be so divergent as to make the parties incompatible in marriage. In order to enforce the award of alimony, you can file a motion for contempt or a motion to for enforcement. The court can order sanctions for the failure to follow the court's order to pay alimony and can even impose jail time for the failure, but that is rare.

It is the relationship that was broken, not the sex. You are divorced. If he is not paying alimony as he was ordered to do, file a motion for him to be held in contempt.