MS SQL Can't drop user since it is DBO

You are trying to drop user from Security/Users and get error something like "Can't drop user who owns schema ..."

Useful link:

http://www.mssqltips.com/sqlservertip/2620/steps-to-drop-an-orphan-sql-server-user-when-it-owns-a-schema-or-role/

Further actions were needed for me.
Go to Schemas, find schema in question ( e.g. db_owner) -> Properties -> change Schema owner to "dbo".
This will release user in question and you should be able to drop it.