Cutting off exchanges from financial institutions will likely be the primary way the government attacks cryptocurrencies. Just look at what happened to online gambling 10 years ago. Online poker and sports gambling were booming. What the US did was prevent banking companies from doing business from gambling companies or intermediaries (e.g. Neteller). Basically overnight, online gambling became a shell of what it was previously. Sure there are still ways around it, but it's enough of a hassle that only the most die-hard players are going to find it worth it.
It doesn't take a huge stretch of the imagination to apply the same strategies to crypto. If financial institutions are prevented from doing business with Bitcoin, et al., it's basically game over, as long as its utility is dependent on buying with fiat currency.