In the meantime I found:
I created a new folder on the web server with Filezilla. I can move the files from wpcf7_captcha and wpcf7_uploads to that folder. After that I can delete wpcf7_captcha and wpcf7_uploads. However, the files (in the new folder) can still not be deleted. Also that new folder has write permission... and of course it is still IIS.
Anyone any idea?