By default, a Mirror profile would DELETE any files that exist on Destination but are not on Source. It wouldn't Copy those files on Destination back to Source if those files are missing (on Source), unless you explicitly configure it to do so.
Those settings are under
Modify > Expert > Decisions-Files
By the way, are you aware of how a Mirror profile works exactly?
http://www.2brightsparks.com/syncback/help/index.html?mirror.htm
The idea of Mirroring is so that your Destination data is exactly the same as your Source, so any orphan files present on Destination but not on Source will be deleted on the next run (unless those settings are customized). Thus I'm not sure why you would think files on Destination would be restored back to Source on the next run...
Those settings are under
Modify > Expert > Decisions-Files
By the way, are you aware of how a Mirror profile works exactly?
http://www.2brightsparks.com/syncback/help/index.html?mirror.htm
The idea of Mirroring is so that your Destination data is exactly the same as your Source, so any orphan files present on Destination but not on Source will be deleted on the next run (unless those settings are customized). Thus I'm not sure why you would think files on Destination would be restored back to Source on the next run...