You can easily delete computers from SecureBackup. Login to your SecureBackup web portal, and click on the SecureBackup tab. Select the computer you want to delete and click on Remove Computer in the File Management box on the right hand side. The computer will be removed from your SecureBackup list, and all files contained within it will be deleted. Note that once you've deleted a computer from SecureBackup it cannot be undone - you will not be able to access those files from your SecureBackup account, unless they are re-uploaded.
When you delete a computer from your SecureBackup account nothing will happen on your local PC - no local files will be deleted at all.
If you delete a computer from your SecureBackup web portal but decide to keep using the SecureBackup Desktop backup software on that PC then SecureBackup recommend that you run an integrity check to make sure that SecureBackup re-uploads any files that were removed. To do this, go to your SecureBackup Control Panel, go to the Advanced Settings tab, select Maintenance and click on Run - SecureBackup will rescan all of your backup folders and re-upload any deleted files.