[Enigmail] Disable PGP/MIME option

[Brian]

Please help.  I need a way to disable the ability for a user to use
PGP/MIME within enigmail.  I have tried adding

   user_pref("extensions.enigmail.usePGPMimeOption", 1);

to the prefs.js.  When I tested sending an attachment Enigmail still
offers the option to send with PGP/MIME.  I really need a way to make
that option disappear.  Please tell me this is possible, and then how to
make it so.

bri