Unchecking Voicemail for the hunt groups settings within the IP Office configuration should do the trick.
If your running Voicemail Pro in Intuity emulation mode, you can also login to the hunt group mailbox and then switch of a feature called Accept call answer (Login, pres 5, then 7 and then 1 to toggle it on or off).
Going the whole hog with VM Pro, you could customise the Leave start point for that hunt group to something other than leavign a message, ie. a transfer.