You can choose to ignore it since 404 is correct since the content doesn't exist anymore.
Or some people prefer to redirect that "removed content" to their forum index. That isn't correct from a content perspective but it gets rid of the 404s. You would use this in your .htaccess in the web root:
Code:
RewriteEngine On
RewriteRule ^(tag|videos)/.*$ http://www.yoursite.com/? [R=301,L]
You can also update your robots.txt file to tell google to stay out of any particular URLs....those with the member and conversation beginnings, etc....