I don't think i have that option with my host. heres example i get this http://phorum-bb.net/forums/viewtopic.php?f=33&t=2 < which is an old page that no long exists. http://www.google.com.au/search?q=site:phorum-bb.net&hl=en&start=30&sa=N
if u mean google is indexed pages u dont want them so u could put a delete record from the webmaster tool and wait might it take day or week or even more and in the main time u could do redirect or u could make error page 404 so when somebody access unavailble page it will take him to the error page that have back link to the forum somthing like that http://www.hostsgate.com/forums/sdfsdfsdf when somebody access wrong page u could do that with Haccess file
to make error page that so easy go to ur Public html u will find a file name .Htaccess open it for edit then add this code ErrorDocument 404 /notfound.htm replace notfound.htm with ur customer error page that u made and u upload on the main directory on public html save it and u will see its work normal when somebody access an page isnt there it will redirect him to the 404 page so he could go again to the forums or anyother place on ur site