You can try using CB Auto Actions on the registration display trigger to add jQuery to the header using a Code action that adds the autocomplete attribute to every input. Example jQuery as follows. Please understand this is a "suggestion" attribute. A browser does not have to respect it. Also formfill browser extensions tend to completely ignore it. Browsers that don't support HTML5 will not validate your sites HTML and may completely ignore the attribute.
Code:
$( '.cbRegistration' ).find( 'input' ).attr( 'autocomplete', 'off' );
I also suggest using an HTML5 Joomla template. If it's not HTML5 then edit your Joomla template index file and change its doctype (should be at the very top) to the following. This will already be the case if you're on Joomla 3.x (probably Joomla 2.x too).