upgrade to a current UBB.threads 7.6.x version.

it's built in since version 7.6.0.

from the 7.6.0 changeling:
Added server status error codes ("404 Not Found" or "401 Unauthorized") for when content doesn't exist or the link is unauthorized to that user/spider. Also added "noindex, nofollow" robot tags to those 404/401 pages.