I just glimpsed at your post, I'm not sure if you solved it. But have you tried going to Transmission's setting options. It should have a setting where you can change where downloaded and incomplete files are stored. From there, you can look where the incomplete files are being stored (you can change it here too if you want so it's easier in the future). After you find out where they are stored, go to where the files are and delete them. Hope I helped, it's been a while since I've used Transmission.