Is it possible to do a 301 redirect at DNS level?
You can't "301 redirect" at the DNS level. You can make it so the domain doesn't resolve or point it somewhere entirely different. Although this should be unnecessary.
Presumably the pages currently return a
404 Not Found status? 404s don't necessarily drop out the index that quickly, since 404s could be a temporary thing and you don't want pages to drop out the index at the drop of a hat.
To send a more definite signal you can return a
410 Gone HTTP status instead. This is a stronger signal that the URLs are not coming back and can only speed up the process.
the pages are still appearing in Google search results.
Is this for a "normal" search or a
site: search? You can often have pages that are returned in a
site: search (yes, it is indexed), but will no necessarily be returned in a "normal" (non-