If you go into the HTMLEditor folder... then go into the "custom" folder, you will see the JS files that control the editor's features for each instance. The two you will need to modify are content_insert_ContentValue1.js and contents_update_ContentValue.js.
Look on about line 35 and you'll see this:
FCKConfig.EnterMode = 'p';
You'll want to change it to read
FCKConfig.EnterMode = 'br';
So, now when you hit enter, it will just create a break tag instead of automatic paragraphs.