In firmware version 8.0 and later, you can cancel share syncing for an unmapped share by clicking the "x" icon (Stop Share Syncing) next to the CacheDrive's name.

If you click the "x" icon, the following dialog box appears.  Click the "Stop Share Syncing" button to stop the share syncing process.

After stopping the share sync, the files that have finished syncing will remain on CacheDrive.  To remove these files from the CacheDrive, manually unmap the share.  To fully map the share, unmap it manually, then map it again.