Msg#: 4314134 posted 11:44 am on Mar 1, 2011 (gmt 0)
A simple redirect using a RewriteRule can fix it, but you need to decide whether you want to redirect just this one exact URL, or a bunch of URLs that follow a similar pattern. If the latter, you need a very exact definition of that "pattern" in order to get the correct code installed in your .htaccess file. The redirect(s) goes ahead of any other WP code in the .htaccess file.
Msg#: 4314134 posted 5:00 pm on Mar 2, 2011 (gmt 0)
well... there's your answer. You need to do two things
1. quit publishing those URLs anywhere on your site
2. send a 301 for all such URLs to the permalink. I believe that nowdays Wordpress should be doing this by default if permalinks are set up right.
So really #1 is your main problem.
One thing, can you explain what you mean by "accessible"? Does that URL end up as the final URL in the address bar, or do you end up with the permalink? And what headers is it sending? Is it currently sending a 301 of ?p=8428