Telnet to the 8300; remove the main site CLAN card information (clear mgc list from the config mode); then reboot the 8300 (reset mgp). If you don't do this, or phonegoober's suggestion of shutting down the network port, you can force the 8300 to go into LSP mode but it will just re-register itself in a few seconds back to the main site.
Susan
"Opportunity is missed by most people because it is dressed in overalls, and looks like work."
- Thomas A. Edison