How can I disable the "Text format" pull-down for users that are creating or editing content?
The absolute easiest way would be to hide the selection using CSS.

I only say this because there's nothing "bad" that can happen if someone does use the plain text format.

Otherwise you're looking at assigning custom permissions and probably using hook_form_alter()  which just seems like it's more trouble than it's worth!

.filter-wrapper { display: none; }