Drupal automatically adds <div>'s and classes around form items. All form elements will have the attribute class = "form-item". How do I override this default behavior?
If you insist on changing it, it's added in the theme_form_element function. You can override it by copying that function to your theme's template.php file and renaming it THEMENAME_form_element (substituting the name of your theme in place of "THEMENAME") or phptemplate_form_element and removing the part that adds the class.
In real experience these are useful classes, but if you want to extremely clear form you can theme each form element or theme form:
Much closer to your task i found here (not only zen theme and not only for user login block): Custom User Login Block on Drupal 6 Zen sub-themes in "Advanced theming" part, see bottom code.
来源：https://stackoverflow.com/questions/5012870/remove-class-form-item-from-drupal-forms