If I go to the Forum General Settings page Miscellaneous Options section and set my forum editor to Enhanced Interface then neither the Enter nor Backspace keys do anything. Anything I type in is all one paragraph.
With Enhanced Interface - Full WYSIWYG Editing the only way around this for me is to create the text in another editor (I.e. notepad) using double line feeds and then paste that text into the forum editor window.
However, if I set the editor to Basic or Standard the Enter and Backspace keys work normally.