Not much for the US to do? So you presume, for example, that punitive sanctions aren't an option? This could finally be the impetus to lay on some "more than just a slap on the wrist" sanctions. I mean, they haven't even banned the export and import of luxury goods yet. There's a long way to go down that road. The big effects come in creating a business environment in which any company that has any operations in the US or Europe is afraid to touch Russia out of fear of massive fines, causing them to over-self-sanction as a precaution (this usually has the greatest effect on sanctioned countries).
There's really no way to ever *totally* stop travel between the borders, and even very strict measures well beyond what we could reasonably expect here would leak like a sieve. But the key is to ruin the ratios in Russia - to devalue their exports, hike the cost of their imports, and raise the interest rate on their borrowing. Which in turn would render most of their business activity except that with a very high profit margin uneconomical, as well as smashing their per-capita buying power.
Yeah, Russia's energy weapon can be used, and that's a big weapon, although it'd aim even more back at Russia (that's the foundation of their economy). Europe has the operational LNG terminals to import enough (even without usage displacement, which would happen en masse - welcome back, coal!) to replace Russian gas, for example, and there's more than enough exporters (Qatar alone could offset it). But Europe would pay out the nose for it - LNG is expensive to begin with and they'd be competing against other buyers all over the world. Again with oil, Saudi Arabia alone has nearly enough reserve capacity to offset Russian oil. But you better bet OPEC will let the price float up a several dozen dollars a barrel in the process. (OPEC really has to be thrilled about the prospect - Russia's been a pain in their side for ages).
So yeah, the EU, and to a lesser extent the US, would really hurt from such a full-out trade war - probably a 5+% loss of GDP in Europe, a brand new recession. I could even picture on the order of a 10% hit in some parts of Europe. But Russia would literally become a third world nation without that income.
A more interesting - and realistic scenario - would be if they can render new investment in the Russian oil and gas industry uneconomical, but not take sanctions too far otherwise. In that case, Russia would continue selling oil and gas from their existing fields - which are largely only set up to export to Europe - and Europe would continue to buy. But Russia's production - and economy - would keep slowly tapering away as the fields aged without replacement. The rest of the world would at the same time have ample time to develop alternative production and Europe would have ample time to adjust.
Whatever happens, I doubt we'd see the necessary support to lift any sanctions on Russia unless they give up Crimea. That is to say, I don't think we'll see anything lifted from Russia for a long, long time. Whatever hits Russia today is probably going to become the status quo for decades.