Is paying prostitutes to perform oral sex considered zina (fornication/adultery), and does its perpetrator exit the fold of Islam, and what is the way to stop these practices?

What you have done is forbidden (haram) and a great sin, even if it did not reach the level of fornication (zina). You must offer sincere repentance (tawbah nasuhah) by ceasing the action, regretting it, and firmly resolving never to return to it. Avoid bad company and places associated with it, lower your gaze, and stay away from non-mahram women. Repent to Allah and seek His forgiveness, and increase your good deeds, for good deeds expiate bad ones. Hasten to a lawful marriage. It was narrated from Ibn Mas'ud that a man committed an act with a woman that fell short of outright indecency, whereupon the verse was revealed: "And establish prayer at the two ends of the day and at the approaches of the night. Indeed, good deeds do away with evil deeds. That is a reminder for those who remember." (11:114) This applies to anyone from the entire Ummah who acts upon it.