Has Nicole been trying to cheat on me again?
But seriously, sometimes these come from random instances besides the one you've mentioned and I can't see how you could block them outside of playing whack-a-mole due to the nature of how the Fediverse....works.
Maybe instance admins could implement some kinda cool off feature before being able to DM another user, maybe the Lemmy devs could like you're requesting outright allow you to block DM's. Thing is, the spammers then adapt and use other methods to desperately try to get you biting their shite.
I had another yesterday
https://files.catbox.moe/lojl62.jpg
that didn't show up until I clicked the [obj] placeholder in Boost and eventually did the risky click shit even thou it could've been CSAM 🤬