The behavior isn't really a bug and it is caused by Sync Services, not Spanning Sync. What you're seeing is called a trickle sync where anytime Sync Services detects a change or addition in one of its sync clients (iCal for example) it initiates a sync. We have a "back door" fix which will turn off trickle syncing in Spanning Sync. To do so, paste this line into a Terminal window:
defaults write com.spanningsync noTrickleSyncing -bool YES
(Be sure to press return after pasting.)
Then, logout of your Mac OS X account and back in again.
Please let me know if you have any other questions.