this is likely tinymce at work again since folks hitting entering an extra return cause tinymce to put in a an extra <p>&nbsp;</p> into the post content…

in the 3.2 release we ar going to try to filter these silly tinymce things out (you dont see them with quicktags)…

but, until then two things you can do…

1) adjust the spacing in your #sforum .sfposttable td.sfpostcontent p class in the forum skin you are using… however, this will make it look squished together for folks who dont enter two returns…

2) adjust the spacing in your theme p class so that when someone hits enter in the editor it has more appearance of a double return….  cant tell you the exact class to change without a link to your site…