As far as I can tell from the Rev2 manual, there is no way to limit the number of voices, in normal poly mode. Unison mode lets you set the number of unison voice from 1 - 16 (in a 16 voice Rev2, which is the only one I am considering), but normal poly mode is always 16.
I'm kind of fond of the voice-stealing in the Prophet-5 and Prophet-6...there is a certain feel when playing slow-release pads, when the polyphony is limited to just 5 or 6 voices.