@pingou : normally you (and some others FAS accounts) were granted list owner right on that list, so isn't there an option for you, once authenticated through fas, to delete the threads ?
I can do that one by one but ideally each list owner would just be able to delete directly archived spam

I've found the option to remove the user from the subscribers, but not to clean the archives. If you have pointers, I'm happy to check if I can avoid bothering you in the future :)

it seems to be a global admin right that is needed, and being list owner isn't enough.
Good to know :)
I so deleted the relevant threads from hyperkitty archives.