You could always just leave them in your list... don't accept OR deny. They'd then have to cancel to re-request (but would do so repeatedly, no doubt). Barring that, you could drop the book from your active list for a bit... maybe add it just before you make a swap request from someone, and then drop it out again.
I keep all RISI swap mails in a folder, sorted by the activity (need to send out, posted, received, rejected, cancelled, etc), and when I'm requesting a book I'll check my 'rejected' folder to see if I've already asked that person for a book (use your email's 'find' feature). If I have and I've not added any new books, then I'll skip requesting from that person. If I *have* added new books (or it's been a few months), then I'll go ahead and request from them anyway.
This was a recommendation from Hecate when I first joined up, and it's worked really well. I don't *think* I've annoyed any people to death, at any rate. ;-)