One thing you might want to try in the future is only agreeing to this sort of thing if you will be working one-on-one with the person requesting it. This forces a cost on the employer for asking for spec work, and not an unreasonable one. Since it's far too easy to ask people to mail stuff in and then not give it the attention it deserves, it also ensures they'll give your work a level of consideration commensurate to the amount of time you put in as well.