Preventing some files like the 404 page being added

[djskinner]

How do I prevent entries like these from being in the sitemap.xml -surely it doesn't make sense:

<url>
<loc>http://www.a1footballfactory.com/404</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>http://www.a1footballfactory.com/500</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>

[djskinner]

This is resolved in 2.2.0 since I can now specify a custom collection.