I don't think it's possible to block memories without mods. I'd assume we'd know about it by now, and I never heard of anything except deleting them one by one, which is too much work and doesn't help that much, because all of the townies still keep creating memories.
Like mentioned, it's very easily stopped by Mastercontroller. Just click "Nraas - Mastercontroller - Settings - Allow creation of memories" to stop the whole process. And this means the entire town. One more reason to love these mods...
And to why I don't want any memories ... I think they are annoying (the icon popping up AND nowadays there's also the added sound, even when notifications are turned off) and a waste of space. I really don't want my 90 townie sims generate memories and make my file larger. It's just pointless in my opinion.