Now Lemmy can implement anything but nothing could ever prevent blocked/muted user to create another account in order to continue harassment.

Not a great argument because the same could also apply to community and site bans.

I think that having more tools to fight harassment is ultimately a good thing, are these tools perfect? Of course not, but they are still better than having nothing.

I think the only way to prevent such issue would be a system which would require to prove identity in some way in order to create a single account. But this is completely against the openness of a federated network.

Indeed it is, plus it doesn't stop those malicious enough to commit a felony just to harass someone but that is neither here nor there, this discussion is about protective measures that can be done before ban evasion.