I have to agree that a polite thanks but no thanks is common courtesy. Unfortunately I know from first hand experience, having written to over 100+ operators in one mail-shot I got less than 50 replies - even one liners saying no thanks - and a number of the ignorant were fully fledge airlines.
Having said that, I did get a personal reply from a well known Egyptian shop owner in London..which boosted him in my estimation - I didn't expect a reply from him, but I got one.
