Well they could always complain to their friends but if ya think about how it would sound ... "We found this photographer to do our wedding and after talking it over we called them back and they had already booked somebody else for our date." ... "damn they must be good if they're that busy!!!!" ... doesn't leave a bad taste in my mouth.
You just always have to keep in mind what your actions will cause people to say.
Ok, so once I turned down a huge monthly contract on a magazine and my method was by busting up laughing at the owner. Now I would never recommend doing something like this. However, in my case, it was for a food fetish magazine and he was WAY too into it. He's trying to conduct a super serious business meeting but the guy is talking about lawn chairs, chocolate sauce and oriental girls ... and going into great depths about what temperature to keep the sauce for the best results and I just couldn't keep a straight face any longer. He never called me back. LOL