AFAIK you can't remove the namespace without doing some kind hack like casting the XML to a varchar(max) and then replacing out the namespace and any prefixes that it uses before casting back to xml again. This isn't a nice thing to do and prone to problems for obvious reasons. You could use a xslt to do this though and would be more reliable than using REPLACE() within TSQL.
Working with namespaces is pretty easy though and all you need is to have something like this before your query:
WITH XMLNAMESPACES(DEFAULT 'http://somenamespace')
If you want to post an example xml document showing what your namespace looks like, we can help further with the syntax so that you can query the xml without removing out the namespace.