Would it be possible for some beings/entities to block core frisk from seeing them. Like a jammer or something. Because to my understanding core frisk can see all timelines/ aus

For all intents and purposes as I wrote it, Core is able to see EVERYTHING, as in that's their whole shtick, their mamma mia 🥺 Removing that for plot reasons is definitely understandable, but also massively out of character. I've seen a lot of writers directly nerfing Core's omnisight because they don't know how to incorporate it into the plot without becoming deus ex machina.

Personally, I did and do enjoy tackling the challenge of writing a godtier superpower while still keeping the character interesting, likeable, and internally consistent. I also understand that not everyone's on board with the concept ("Well I don't want someone's OC to stalk my AU just because the author said they can!") and honestly, there's no correct answer to that. Whose word overrides whose? The creator with an AU that cannot be seen, or the creator with an OC that can see all? If someone takes your creation and changes it in some pretty fundamental ways that go directly against what the creator has established, who's correct - the creator or the author?

If you don't really care about any of that and just want answers to your question, the answer coming from Core Frisk creator is - You can't prevent Core from seeing someone or something, but you can for example stop them from appearing nearby you or in your timeline by jamming the bodies' frequency; you could trap or destroy their body everytime they do try appearing; you could write a timeline in which Core appears too inconsistently to matter; you could make people not trust some creepy grey child thing that suddenly comes around with urgent warnings; you could just not use Core in your story if you think they'd get in the way of the story you want to tell in ways you don't like.