Tradesmen of any description.
If you don't want my business, just fucking say so - don't leave me hanging around waiting for a callback and/or say that you'll come round at such-and-such a time and then not bother turning up. Not only does it waste my time, it also gives a poor impression of your profession and, above all else, it's bloody rude and inconsiderate.
Plumbers are, by far, the worst offenders in my experience, closely followed by electricians.