Easy.

Step 1) outlaw sale of bitcoin to anyone other than government

Step 2) demand all sales of bitcoin be completed by a certain date

Step 3) declare any unsold bitcoin held by us citizens property of US government

Step 4) theft from US government is a crime that will follow you everyone around the globe…merely leaving US isn’t protection against prosecution for selling stolen government property

You are correct that this doesn’t guarantee they’ll get my bitcoin. But it does guarantee the exchange rate to the US dollar temporarily collapses.

Will it make bitcoin held outside the US to increase in value? I doubt immediately that it would.

The only way to refuse to give up your bitcoin is to have an income from conventional sources and be content with the possibility that you’ll have to pass on your bitcoin to your kids and never enjoy the fruits of your investment yourself…such a process could take 40 years or more to play out.