Theres a free app on the market called AutoSync Account Activator that lets you select sync time intervals for all of your accounts. I'm sure you can use this to completely disable their sync. You can also use it to sync over wifi only. Make sure that your contacts application uses its own database though. Many contacts apps are still using the google contacts database and therefore still need the sync functionality. If you are not sure, try to create a new contact in the google contacts app and then see if it shows up in your 3rd party contacts app. If so you should leave contacts sync at maybe a 24hr interval.