How remove a user from a role?

I've been using Xaraya for ages but I don't recall ever needing to  
remove a user from a certain role. In this case a user has very  
mysteriously become an administrator and I can't see a way of  
removing her from the admin role. I don't want to completely delete  
the user and she also belongs to another role. How to I remove this  
user from the administrators role? I can't seem to find any way of  
doing it in the Roles module. Maybe I'm just being a bit dim this  
morning.