When you create an RSS block, it stores the RSS link / URL in the nuke_blocks table, and, unless you're using nukePIE or RavenNuke 2.20 or higher, it stores a cache of the RSS contents also in the nuke_blocks table (nukePIE stores the cache as a file in the cache directory).
As Guardian said, you can delete the record in the nuke_blocks table that has the ancestry.com feed using a database tool like phpMyAdmin.
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