I'm sympathetic on the point about being polite, and I don't like to be rude to people if they have responded sensibly, politely and have clearly read what I am after. So to decide on a response, I tend to split them into:
Totally unsuitable bids (ie wrong services, bidding more than their usual rate, outside age range, bid without message or clear copy/paste message, or if it seems they have not read the booking): these get totally ignored, and deleted as soon as I see them. In my book, they have been rude or are chancing it, so I don't feel impolite for ignoring them.
Bids which are fine but I simply don't want to meet the girl (usually I just don't find them that attractive, but they offer all that I have asked for and are not being pushy) - I tend to leave these until I close it, then a very quick "thanks, but no thanks". No point in dressing it up if you don't ever want to meet that person.
Bids which I would happily accept: these get a simple acknowledgement when I first see them, then, to those who are ultimately unsuccessful, they get a very straightforward "thanks again, but I have decided to meet a different girl this time. Perhaps we can meet in future". Once or twice this has led to a meeting another time - and it. Usually means they will bid again, so it kind of keeps your options open.
Bids by email: i don't mind if someone has a question about the booking, but you get one or two who for some reason bid by email, which makes it hard to keep track and you're rarely sure how much money they expect. Not that long ago I had one stupid cow email me ages after I posted the RB saying "why don't you tell me how long you want to see me for and how much you want to pay". My perfectly polite response that all of that was on the reverse booking and if she was interested, she should place a bid generated no further email from her.