I keep hearing and reading that they can't do anything about Bitcoin, that they can't enforce what is in Elizabeth Warrent's bill. But nobody has a good argument, if any.

Sure they can't stop Bitcoin. The software is on thousands of servers around the word. Developers around the world are working on Bitcoin applications, some of them anonymously. The US government can't have all of the arrest and extradited. 

But they have control over financial institutions and the businesses who depends those institutions. Also businesses have to register, obtain licences, provide detailed accounting, file tax. 

Individuals transact with both. So they don't need to actively enforce the rules at the individual level.

All they have to do is to set rules for financial institutions and businesses, and make sure that they do follow the rules. 

As most of the transactions are B2B and B2C, they have effectively enforce the law and regulations on the vast majority of transactions.

And don't forget that the individuals who don't follow the new legislation, and get caught, they won't only face criminal charges for that, the IRS will have their own charges for not reporting the disposition of Bitcoin and the payment of the taxes on capital gains.

We'll see how many of those who say they will not comply, will actually not comply. Which ones will be ready to potentially loose it all: assets, family, and liberty.

Bitcoiners represent a small fraction of the US population. The government wont loose much in terms of enforcement if a small portion of those Bitcoiners is not compliant.