Just nuke the thread and be done with it.
If everything was that easy... I would love to get it trashed but you need a proof that those accounts are really connected to the service otherwise you are just shooting in the dark.
If they are not reporting spam they are very likely using bumping service. It doesn't need to be trashed, just locked for few days. If "they do it again", it can always be locked for more days and this would make payed bump services pointless.
It can be done like this:
1) report thread which you suspect it is being bumped by "fake" accounts
2) don't report spam accounts
3) let moderator do the rest of the job
If they continue doing so - trash it. Less work for everyone.