You can block specific PCs from using a license by logging into your account at https://accounts.flashbackrecorder.com/Login.aspx and then using the Manage Licence Usage option on the Your Licences page.
When a PC is blocked from using a license, it is a permanent ban. If you just one to unregister the license from a PC, the Remove option should be used instead.
If you have blocked a PC by mistake, email us at support@bbsoftware.co.uk to get the block removed.