Not in membership caml query

Hi

I have a SharePoint List with an owner column ( type of Person/group)  that contains the SharePoint group that own the item.

Now I would like to show to user in a webpart only items that he does't own. I have to use a caml query that must contain membership clause but i don't see how to use the negative form of this query : 

 query.Query = "<Where><Membership Type='CurrentUserGroups'><FieldRef Name='BelongsTO/></Membership></Where>"; 

How to say " items that don't belongs to Me " in caml language. 


Any help will be gadly appreciated.