Change xxxxxxx to whatever you want and make sure to set a cookie on your members main page named xxxxxxx or whatever.
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*your-domain.com/ [NC]
RewriteCond %{HTTP_COOKIE} !(^|(.+;)*)id=xxxxxxx(;.*|$)
RewriteRule /*
http://your-domain.com/ [R,L]