Forcing OpenID login instead of Drupal own authentication
I simple solution would be to alter the login form to remove the username and password fields.

Here is some sample code to get you started:

/**
 * Alter the login form page
 */
function MYMODULE_form_user_login_alter(&amp;$form, &amp;$form_state, $form_id) {
  _MYMODULE_alter_login_form($form);
}

/**
 * Alter the login form block
 */
function MYMODULE_form_user_login_block_alter(&amp;$form, &amp;$form_state, $form_id) {
  _MYMODULE_alter_login_form($form);
}

/**
 * Function to change a bunch of things in the user_login form (both the block
 * and login screen).
 */
function _MYMODULE_alter_login_form(&amp;$form) {
  unset($form['name']);
  unset($form['pass']);
}


Unfortunately I don't know of any contrib modules that will do this for you; the good news is that I have wrote 95% of the module for you. ;)