How to remove a prefix with hook_form_alter? Looking at the source of a module, it is doing this:

$form['buttons']['back'] = array(
  '#prefix' =&gt; t('or'),
);


Now I want to remove that prefix; I tried the following code, but it doesn't make any difference.

function mymodule_form_formid_alter(&amp;$form, &amp;$form_state) {
  $form['buttons']['back']['#prefix'] = ''; 
}


I tested the following code, but it doesn't seems to work; the prefix is now 'testingor'.

function mymodule_form_formid_alter(&amp;$form, &amp;$form_state) {
  $form['buttons']['back']['#prefix'] = 'testing'; 
}


I tried the following code, but it doesn't make any difference.

function mymodule_form_formid_alter(&amp;$form, &amp;$form_state) {
  unset($form['buttons']['back']['#prefix']) ; 
}


What am I doing wrong?

The form ID for the form I am trying to change is "commerce_checkout_form_checkout" which is used from the Drupal Commerce module.
In your hook_form_alter the following should do the job and has the benefit of leaving the element there if required.

$form['buttons']['back']['#access'] = false;