Go ahead and use the candle. Just be careful and don't light up your feathers. Turn the arrow constantly over the flame, just low enough to heat the plastic and make it soft, not in the flame itself. When it gets soft enough, you can peel the nock off with a pair of pliers and file off the old glue with an emory board.

For me, it's a lot easier doing it that way than trying to cut the old nock off with a knife and risk messing up the taper.

I like the Bohning Classic nocks, myself, but that's what 3 Rivers says comes on the Barta arrows. I'd bet you've got size 11/32, so I'd suggest trying the 5/16 instead. The 5/16 ought to have a smaller throat than the ones you've got.

edit: Or do what JimPic suggests. Another option is to just re-serve the string with a larger diameter serving.