The ICO put a market of live human organs as example.
In same manner, even if I would click that I agree, that your site is designed for US privacy laws and not for people under GDPR protection, it would be the same as you would warn me, that I will be your slave before signing and that I can just walk away and don't take the car. But if I take the car, the contract would still be void. I don't think that this would fly.
The problem is not in the GDPR requirements but rather in right to privacy as fundamental human right and GDPR is just an advice how to respect it - it is actually a free help.
What you want to avoid is something much bigger than the checkbox on your site or ip blocking, check here "The Bill of Rights":
https://en.wikipedia.org/wiki/Fundamental_rights
This is something you shouldn't even think to violate, not to EU or US users. Or anyone else.