To turn sync on start off enter the following line in a terminal window:
defaults write ~/Library/Preferences/it.holtwick.mac.favs HOPrefSyncOnLaunch -bool NO
Favs is sandboxed i.e. you find everything inside of the following folder:
Favs uses some folders to store additional data. If you want to remove Favs completely from your computer you should also remove those folders. Please not that you are doing this on your own risk! It is always better to have a backup around in case you need to restore anything.
The database is stored in
~/Library/Application Support/Favs which you should not touch at all.
All downloaded images are cached in the
~/Library/Caches/it.holtwick.mac.favs/it.holtwick.mac.favs which you can delete any time since they will automatically restored as needed.