Is there a way to disable username for logging in?
I don't know of an existing module that does this, but it would only take a few lines of code in a custom module to get the effect you're looking for:

function MYMODULE_form_user_login_alter(&amp;$form, &amp;$form_state) {
  // Add an extra validation step to the form.
  // The user is actually authenticated during the validation stage
  // so we have to make sure this validate function runs before any others
  array_unshift($form['#validate'], 'MYMODULE_user_login_validate');
}

function MYMODULE_user_login_validate($form, &amp;$form_state) {
  // Make sure the provided username field is in the form of an email address.
  // If not we can safely say the user shouldn't be allowed to log in,
  // regardless of whether or not a user exists with that username.
  if (!valid_email_address($form_state['values']['name'])) {
    form_set_error('name', 'The email address you have entered was not found in the system.');

    // Empty the name variable so further validation definitely fails.
    // There are probably better ways to do this so be creative :)
    $form_state['values']['name'] = '';
  }
}