When you are trying to remove user from a database, you will get:
The database principal owns a schema in the database, and cannot be dropped. (Microsoft SQL Server, Error: 15138)
There is a easy fix for this.
First see what Schemas the user principal own's:
USE DATABASENAME;
SELECT s.name
FROM sys.schemas s
WHERE s.principal_id = USER_ID('domainusername');
Then use that database to the following query:
ALTER AUTHORIZATION ON SCHEMA::db_owner TO db_owner;
Now you can delete the user and add it again.