If you click on the "Remove this swap from my active list" next to the relevant swap, you will then go through the process to move it into Uncompleted swaps. I think (from memory) it asks you for a reason why you're moving it into uncompleted swaps but am not sure whether it also asks you...

I agreed a swap some time ago and the other person has received my book. Sadly she contacted me to explain she'd suffered a bereavement in her immediate family and hadn't managed to send my book. I told her not to worry about sending it in the circumstances. How do I now remove this from my active...