Yes I have, ages ago, it was very good but I just couldn't be bothered. I think it gives you a list of why you don't want to swap?? Just say you don't like any of their books. I think this will happen a lot so people won't be offended.
Have to say, I prefer BookMooch as, instead of having to find someone with a book you want, you just get a point for sending a book, that you can then 'spend' to get a book from anyone else on Bookmooch.
Completely agree re. BookMooch - the only prob is that it is quite heavily US-based, and while I love US fiction, a lot of people won't post outside the US as there is no surface mail any more so it can cost more than a new book to post internationally.
People reject my swaps on RISI all the time. I am hardened to it . It's the way the site works and if you dare to suggest it might be nice if... you get slapped down pretty quick.
I use it quite a bit (I'm a superswaper and I've only been using it since about November). It's absolutely fine to turn down a swap using the "I don't want to read any of their books right now" button. I think I probably accept only about half of the swaps I'm offered, possibly less. And I probably only have a third of my offers to swap accepted.
There are bound to be other copies of the book available so don't worry about turning a swap down or having your swap turned down.