Remove Plain Text/HTML dropdown for mailing list

I added a Mailing List block on my site and I would like to remove the Plain Text/HTML dropdown box. Is this possible in the Admin section or will I have to edit the template file?

Also, if I remove it from the template file, what is the Default setting? Can I specify what the Default option should be?

As of version 2.1.1 it is not possible to remove the drop-down box via admin.

A solution can be found in the following thread: