g1smd - 11:30 pm on Dec 20, 2010 (gmt 0)

RewriteCond %{HTTP_HOST} !^example-B\.com
RewriteRule ^widget/ /filepath-which-is-known-not-to-exist.lmth [L]

If the above code is located at example-B.com/.htaccess and you request example-B.com/widget/ the server should return a 404 header and the 404 error page.

Clear browser cache before retesting.

Do not edit the "/filepath-which-is-known-not-to-exist.lmth" bit. Leave it EXACTLY as it is.