Do you also block all US IPs to avoid patent infringement claims? What makes you think the GDPR is broader in scope than the patents the USPTO hands out? In the patents' case, there is even a direct financial incentive for companies to sue you, whereas in the GPDR the interest of the regulatory body is primarily compliance.

That won't completely protect you, it would only greatly reduce your surface of exposure; the eu user could still reach you via a vpn or simply sign up while vising the US.

Ask if user is European, and if they answered yes, deny them service. If they lie, then you have plausible deniability to mistreat their data.

Users of free services will just set their country to non-EU, and continue to consume the services. GDPR will gave achieved nothing in that case.