"a" is not a namespace, it's a prefix.

Are you trying to strip the prefix without changing the element's namespace so

<a:houses xmlns:a="...">
   <a:house>
      ...
   </a:house>
   ...
</a:houses>
[download]

becomes

<houses xmlns="...">
   <house>
      ...
   </house>
   ...
</houses>
[download]

Or are you trying to change the namespace of the element to nothing and thus stripping the prefix so

<a:houses xmlns:a="...">
   <a:house>
      ...
   </a:house>
   ...
</a:houses>
[download]

becomes

<houses>
   <house>
      ...
   </house>
   ...
</houses>
[download]

(Either way, can't help you.)