Reverting an entire sync is not possible which is why we always advise to have a backup of your database and update it now and then. However, do note that PieSync will NEVER delete contacts.
Did you mistakenly sync too many contacts into your Google Contacts? You can use Google’s built in “roll back” feature to just get back to a pre-sync state.
Sync too many contacts into your Command? Fear not. Filter your contacts by the “created” field and you’ll be able to identify those contacts that were created by the sync. Of course, make sure you turn off the sync beforehand.