You are disabling just your select, but chosen renders it as divs, and spans, etc. So after disabling your select you need to update the plugin to make the select widget disabled too. You can try this way:

$('#foobar').prop('disabled', true).trigger("liszt:updated");

//For non-older versions of chosen you would want to do:

$('#foobar').prop('disabled', true).trigger("chosen:updated");

I found the information here

Fiddle

Once you update the widget all it does is it unbinds the click or other events on the plugin and changes its opacity to 0.5. As there is no real disabled state for a div.