I found a way to delete up to 400000 headers from the DB, and so, step by step, I was able to remove the unwanted headers without using "Delete all posts from Poster" because I really, really need the collections of 10 year old headers..
But it needs a powerful PC.
1. In Options set "Display Age:" to a few days.
2. Invent & use some search REGEX to restrict the number of posts displayed somehow. I used 16 search patterns:
search in Poster Field: "(?<first>0[A-Za-z0-9]{8})\s<\k<first>@\k<first>\.\k<first>>"
...
search in Poster Field: "(?<first>f[A-Za-z0-9]{8})\s<\k<first>@\k<first>\.\k<first>>"
3. Open a search window and use one pattern (this step needs a few Gigabytes memory)
4. Hit CTRL-A ... wait .... hit SHIFT-DEL .... wait some minutes (in this step I used process explorer to find out when the delete's really finished (=no CPU+no I/O) (this steps need some more memory + 100% single core usage and a lot of I/O for several minutes)
5. close the search window (<- it appears that not all memory is freed in this step) and repeat with the next search pattern until all groups all clean.
6. restart newsbin (to free memory)
7. Use "Post Storage" -> "Compact Group" to reclaim the space the deletes have freed.
..come un gatto vorrei vivere..