The implementation of the blocking mechanism is IP addresses/ranges, which is imprecise (to say the least). But now you have to worry about abusive users bypassing your technical control by obscuring their IP addresses. So you block all IP ranges that implement e.g. CGNAT, VPNs, 464XLAT.
So now you're mass-blocking access to millions of people who have never shown any inkling of malicious intent due to rational technology choices by their service providers or due to a reasonable desire to protect their personal privacy.
If you're OK with blocking users in such an entirely capricious and arbitrary way, why not just insist on registration?
Your argument is as flawed as saying we shouldn't have email because spammers must be blocked.