The simple answer: make sure that a voice isn’t playing when you steal it…

If a voice is stolen while playing a sound, the sound it was playing will end abrubtly. This equals an enveloppe with release set on 0 and thus producing a click.

To prevent hearing clicks you have to play with poly config and exclude tracks playing sounds with long release-tails from the poly-pool. (Or shorten the notes, so that they end before being stolen)