If you don’t want to use markdown, just don’t use it.
I wanted to add an editor, but there are all sorts of problems doing that, this was the best I can come up with that won’t take me too much time.
As for the note about styling, that should only be visible if the message is editable by the user. Surely if they can edit, then giving the opportunity to style the message is a good idea.
Hmmm…I just had a thought. If the message isn’t editable by the user, I should be able to have it display with the formatting rendered as it will appear. Would that work for you?