It might be possible, but I don't know how. The wysiwyg editor markup is entirely handled by the TinyMCE javascript plugin... try googling for something like "prevent tinymce from outputting p tags". You probably won't find specific instructions for Concrete5, but if you look in /concrete/elements/editor_config.php, that's where the TinyMCE javascript code is... might be able to figure out what you need to do there.

Best of luck.

-Jordan

I know it's an old post which I stumbled upon, but in case anyone else is wondering, you can do it like I described previously here:
http://www.concrete5.org/community/forums/usage/stopping-p-tags-aro...