Handling Non-printing Characters

An XML server side process is getting a text that has been pasted in. 
Within the text are non-printing characters which 
are apparently handled as UTF-16 and the XML processor 
on the server replaces with character entities. On 
the client side, the XML processor says these are not 
legal characters and is apparently treating them 
as UTF-8.  What is the best way to handle these?

Is there a way on the client side to force an encoding?  

Is the way out to clean and strip these before handing 
them to the XML processor?

This is .NET, so if that helps....

len