If you're based in the US with no presence in the EU the pragmatic approach may be to be sensible from the start about personal data handling but to ignore the GDPR, and not to bother blocking anyone.
What worries me is that it’s still early days for enforcement, but they’ve already handed out something like US$1.6B in fines. I assume they will only get more aggressive with this over time.
You cannot be expected to comply with the laws of every country in the world.
Do comply with Australian laws and relevant laws and regulations of countries to specifically target and ignore the rest.
If you see traction in the EU then you can start paying attention to the GDPR.
Unfortunately our government advice appears to be that we should comply if we "offer goods and services in the EU", or "monitor the behaviour of individuals in the EU.": https://www.oaic.gov.au/privacy/guidance-and-advice/australi...
I have no idea what would actually happen if they decided to fine an Australian entity.