good idea. I allways have a big amount of files that will never finish. It happens that you start to download and after the other peer goes down (damn too those who do not care about running upoloads)... you never find another peer.
Propably a button to clear all downloads <50% would be fine or maybe better a button to mark all downloads older than x days (7< x < 14) . After the files are marked you can take a look on them, add or remove more and finally delete them.
Currently I do this manual every week.

cya, Moak