How exactly do non-mining nodes enforce the protocol? They can say it is invalid, but what can they do about it?
Duh, they can ignore invalid blocks and ban the IP addresses of anybody who sends one. To the Bitcoin protocol, invalid blocks simply do not exist. To full nodes, invalid blocks simply do not exist.
ir.hn, stop spewing the same gibberish simultaneously over multiple different threads whilst ignoring, twisting, or dissimulating around all the thoughtful explanations given to you by smart people. (Smart people,
please stop wasting your time trying to explain to someone self-evidently ineducable; IMO the only task is to provide sufficient answer that newbies who happen across these threads wont be misled by ir.hns disinformation campaign.)
So what is the net effect if all non-mining full nodes ignore a block they think is invalid and no longer acknowledge blocks from that IP address? It would just be as if those nodes no longer exist, and as I've said before, bitcoin worked before anyone ever invented the "non-mining full node". Unless you can stop other miners mining on the block you think is invalid, you have no power. Or you actually mine and win the next block, then what you say matters.