VBO (Views Bulk Operations, a great module) does an admiral job. I could only get it to work for about 250 at a time; 500 at a time didn't time out but instead weirdly caused a validation error. Also of note: Filtering by user, the autocomplete has anonymous as Anonymous even if you've renamed as Visitor or such. And there is no "anonymous" role to filter by so if going by role, it has to be an "everything but everything else" filter.
http://drupal.org/node/839850
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