What version of ACF are you using?
For the most part the ACF WYSIWYG editor works the same as the standard WP WYSIWYG editor, meaning that it automatically adds <p> and <br /> tags where it thinks it needs them.
Here’s an older topic where the developer talks about this and ways that it can be removed. http://support.advancedcustomfields.com/forums/topic/removing-paragraph-tags-from-wysiwyg-fields/
