When working with multiple field joins between two tables in Query Builder and you want to delete or modify a Join line you are forced to delete every join line you created before the one you are trying to delete or modify. Even if you click over your desired relationship the selected join line is always the first.
It would be nice if you could delete the desired Join Line for deletion.
You can also incorporate edition in the Join Properties window. Here you could include option for deletion or maybe a drop list for the fields in each table to modify the relationship.