You can define an Action maapping without a name attribute, which means it has no reference to an ActionForm. If you do so, null will be passed to the execute method as the ActionForm parameter when the method is called by Struts.
However you cannot use such an action in an <html:form> tag. Any Action referenced in this tag must have an ActionForm bean attached. Whether it's a DynaActionForm as described by Roshani or a regular ActionForm doesn't matter, but you must define one.