Dragging files to the Boxcryptor(X:)/Dropbox folder automatically syncs them unecrypted to Dropbox.
Once the unencrypted files are uploaded to Dropbox, all that can be done is to use Dropbox's "Permanently delete" option, which according to their privacy policy:
"Please note, however, that there might be latency in deleting information from our servers and backed-up versions might exist after deletion"
Since you can encrypt files only after dragging them to the volume (at least as far as I know), doesn't this essentially make Boxcryptor useless unless you:
1. Drag the files to the boxcryptor volume, outside of the folders for any cloud services
2. Then you encrypt them
3. And then you drag them to Dropbox or Skydrive or whatnot?