To ensure they stop ripening in the fridge, dry the unripe avocados, and vacuum bag them, or place them in tightly closed bags while excluding as much air as possible. Store them like this for four to five days days if the fridge. Then transfer them into a typical not quite airtight fridge style container. Make sure they stay dry
By restricting their ability to absorb oxygen, the CO2 they expel will be re-absorbed. This CO2 "shock treatment" is what is used to sea freight avocados around the world
Be gentle when handling so as not to bruise them. Any bruises will start the ripening process
Buy regular just unripe avocados, that look in good condition, and have not been roughly handled
Most avocados ripen in two to three days after being removed from the fridge, if left sitting in a warm and sunny place