How to Undo the Last Sync on an iPhoneby Warren Davies
Although there is no built-in feature allowing you to undo an iPhone sync, you can do so by restoring a previous backup. ITunes maintains your old backups indefinitely, so all you have to do is identify which backup you made prior to the most recent sync and then restore that one. In effect, this will undo the sync and restore the iPhone to the state is was in previously. Unfortunately, if you don't have a prior backup to restore, you won't be able to undo the sync.
Connect your iPhone to your computer and launch iTunes.
Click the "iTunes" icon in the upper-left of the screen and then click “Preferences.” This brings up the “General Preferences” dialog window. Click “Devices” from the main menu.
Locate and select the backup that you want to restore from the “Device Backups” list, based on its displayed date. Close the General Preferences window to return to the main iTunes screen.
Click your iPhone's icon in the upper right corner of the screen and then click the “Restore Backup” button under the “Manually Back Up and Restore” heading.
Select the backup that you identified in step three and then click “Restore.”