I've always been able to get them out with just a little heat but then again I always just used a little hot melt to hold them in place.
You could try the same method used to get inserts out of carbons. Find a drill bit the same size as the inside diameter of the arrow. (see my note below before trying this)
Insert the drill bit, flat side towadrs the nock and swing the arrow in a downward motion like a whip. The force of the drill bit hitting the nock ring should knock it out.
NOTE: Because there is not much of an edge on the nock holder for the drill bit to catch on it may just loge itself in there, you may want to try this slowly to see if it will work, it may just lodge the drill bit in the nock holder.
Otherwise try to use a pair of pliers and pull them out, just replace them if they get chewed up.
That's all I can think to try at the moment.