Adrenocaine sorry i got the spelling wrong it is used for dehorning cattle, but can be bought over the counter.
Inject sub/cut into the tail around where you are going to put the ring on we inject about 2ml in small doses, so we flood the tissues that will be squeezed by the ring ( this applies to the scrotum as well ).
Leave for 10 mins like at the dentist, so the tail goes numb then apply the ring.