I'm confused. You want to have more liberal "visitation", parenting time or you want to legally end the availability of that time?
Either way, the answer is the same. You have to go to court and get the order changed if you want to make a legal change.
If you, on your own, allow for more liberal visitation time, you are opening the door for him to go back to court and ask for more involvement.
And you need to be concerned about safety. Because he has not completed drug rehab I'm guessing you want to eliminate the requirement for visits.
Returning to court is not only expensive but I believe not in the best interest of your children. They may feel that you are the one preventing time spent with their father. You become the "bad guy." When their father is allowed visits but does not visit they will then know that not visiting is his choice.
If your difficulty is wanting them to visit their father but the cost is prohibitive I don't know of an answer in light of their father's drug addiction. Again, this is more the father's problem to solve than yours. He chose to move to Alaska.
If you're wanting your children to have more contact you can arrange that thru phone calls and mail but only if their father also wants more contact.
It is best when children have a relationship with both parents but this will only happen in a healthy way when both parents are co-operative with each other.
If not having a relationship is the result of their father's failure to comply with the court order the children are able to see for themselves what sort of man/father he is without getting confused by your wanting to prevent contact.
Who their mother and father are and what sort of relationship they have with them contributes to their sense of who they are. It's important to keep the boundaries clear. This means that even tho you may facilitate contact you allow the father to be who he is and allow the children to experience for themselves who he is. This also means that you present as neutral an opinion of him in front of the children as you can. When one parent criticizes another parent the child usually takes on that as a criticism of themselves. This is an internal process that often doesn't manifest itself until the child reaches adulthood.
If you were to put a stop to contact you become the one in control which gives the father and your children to blame you.
If you set up visits that are inappropriate because of his previous lack of involvement and especially because of his drug addiction you also become the one to blame for the poor relationship.
I suggest that the best thing that you can do is, as others have said, get on with your life and let your ex do what he will do while protecting your children from the results of his drug addiction. When they express wishes to see him you can just say, "that's up to your Dad." and sympathize with their loss.
It is a loss to not know their father. I would hope that their step-father is able to take on the father role. But he can never take the place of their birth father and it's important that he not try to do so.