I think it’s okay to block if the mass messages are really getting you down. I also think it’s good if you did to explain to her why as you plan to. Someone (who I was close to) once unfriended me for but didn’t tell me and that was the worst part, wondering what I’d done wrong. Honestly I would be okay with someone blocking me if they explained why beforehand, although of course can’t say how your friend would react. I think it’s emotionally intelligent of you to think about telling her why.