The tpl_searchform() method has a $ajax parameter so you could disable it in your template (see
http://xref.dokuwiki.org/reference/dokuwiki/inc/template.php.html#tpl_searchform). I guess there'd also be a way to disable it through a userscript but I'd have to dig in deeper in the code figure it out right now. Maybe someone else can come up with a simple script.