I've never heard of a plug-in unit, although it wouldn't surprise me if something like this exists. There are however several collars that emit a high frequency sound that you can buy. You can check Foster's & Smith to check prices (do a google search). They also have collars that spray water and well as give a mild shock. All the collars may work just fine or they may not work, but it is the only thing short of removing the vocal cords that I know of that may work without behavioral modification.