Removing a user that owns an object from a database is impossible, so if you want to remove this user from the database, you'll have to change the schema's owner to another user. You can do that with this command:

ALTER AUTHORIZATION ON SCHEMA::WriteSchmeNameHere TO WriteNewOwnerHere

After modifying the owner there you can drop the user and there should be no problems at all accessing the objects on that schema.

Adi