If you try to drop a user that owns a schema, you will receive the following error message:
The database principal owns a schema in the database, and cannot be dropped.
In order to drop the user, you need to find the schemas they are assigned, then transfer the ownership to another user or role
FROM sys.schemas s
WHERE s.principal_id = USER_ID('joe')
-- now use the names you find from the above query below in place of the SchemaName below
ALTER AUTHORIZATION ON SCHEMA::SchemaName TO dbo