Is a UK divorce decree enforceable in the USA?

There are many things that you would need to do and issues that you would need to clear up before you could stop paying alimony. First, in response to your question as to whether the UK divorce decree is enforceable in the USA, the answer is that it can be enforceable in the USA if it is registered with the court of competent jurisdiction in the United States. Second, mere belief that your ex-spouse is no longer residing in the United States (and as you mentioned, your ex-spouse must reside in the United States as a condition of receiving alimony) cannot justify a unilateral decision on your part to stop paying alimony. This means that after you have duly registered your UK divorce decree in the United States (and in Utah) with the court of competent jurisdiction, you would then need to file, depending upon the language in your decree, either a petition to modify the decree of divorce or a motion to terminate alimony based upon your ex-spouse's failure to comply with the alimony conditions of the decree of divorce. Only after the court orders that you may terminate the payment of alimony would you be legally permitted to terminate the payment of alimony. However, the court would likely grant a motion on your part to suspend payment of alimony while the case was pending, so that you do not end up paying alimony month after month after month only to have the court determined that you should not have been paying all that time, yet being unable to recover that money from your ex-spouse who would most likely have spent it.

Yes. A UK divorce degree is enforceable in the US. As to the conditions of alimony. If they are not being met you do not owe it so write a letter with your next check ask him for specifics as to his compliance with the conditions if he does not respond stop paying. (or better yet pay into an escrow account just in case he challenges it and you loose)