First you will have to determine the schema's owned by a given user in order to drop the user.
Execute the following query to determine the schemas owned by the user named ‘user1’ .
SELECT name FROM sys.schemas
WHERE principal_id = USER_ID('user1');
When you run the query above, suppose you get db_datawriter as the schema owned by user (in this case user1)
Next step is to change the owner of the schema db_datawriter to some other user (e.g. dbo):
ALTER AUTHORIZATION ON SCHEMA::db_datawriter TO dbo;
You will have to remove all the schemas to drop a db user. When the user has no schemas owned you can drop it.