The Russians can withdraw, but not close the ISS
Roscosmos is committed to support the ISS through 2020. According to the 1998 NASA-RSA agreement (MOU), the RSA can withdraw anytime they want after that.
At current plan, the NASA commercial manned-LEO-lift ability will come online in 2017. A second commercial vendor to come online by 2018.
Orbital-Antares (5) & ULV-Atlas (2+) claim to warehouse enough Russian-sourced equipment to last years. SpaceX-Falcon and ULV-Delta use NO Russian-sourced equipment. Both Dragon and CST100 attach to their rockets with the same technology and controls, so CST100 could launch with Dragon rockets (whenever THAT gets tested).
ESA Ariane 5/6 would certainly be considerations, as would JSA H-IIA/B.
Boeing's SLS isn't planned for use with the ISS. It comes online by 2020, but is priced for more distant targets.
In short, permanent separation of Roscosmos from ISS is a horrible shame. It might bring about the closure of ISS after 2020. There are other Space Station plans that could take advantage of the opportunity by that time, though.