Try and quit.
Last time I did, they retroactively declared nearly all of my purchases since joining the club to not be elligible for paying off my commitment upon joining and presented me with a big bill and an e-mail threatening legal action, and this is after I'd purchased more than a dozen books through them.
It took me weeks of long distance phone calls before I could get it resolved, and it was made very clear to me that they felt they were doing me a favor by not taking me to court.
I thought this was an anomaly, since I used them with great satisfaction in the 1980s and 1990s, when I didn't have ready access to book stores (living overseas) and they were great. Now, I find out, that lots of people have stories like mine, with the SFBC claiming "members" owe them all sorts of things the members believed to have already been taken care of.