The moment when they have reached the limit is described by Nephi as "fully ripe in iniquity." That means they will no longer even listen to the truth. They have completely closed minds. It would do no good to extend them further opportunity, because they will not take any advantage of it.
How humble it is for the Lord to be willing to accept the reluctant, tardy and slow to repent. Nevertheless, He is willing to accept even them. He suffered for all, and will redeem as many as will come to Him. Initially, He won't destroy them with the wicked. Ultimately the outcome will depend upon how committed they are to the process of repentance. For to repent is to come to Him. They decide if His open arms will be where they finally embrace Him; of if they will stand afar off and think it too hard to surrender their sins and go further.