Not much can be done to stop Putin from invading Ukraine and Obama has basically given Putin the green light with his comments and recent history of weak foreign policy. Putin has said that the demise of the Soviet Union was the worst thing to happen in the 20th century and he doesn't think that Russia can be a superpower without the absorption of Ukraine.

With the Russian parliament giving a 100% vote for military action in the Ukraine the line has been crossed and I don't see much anybody can do to protect Ukraine sovereignty and sanctions will be after the fact.
There is The Budapest Memorandum Pact which Clinton, the UK, Russia and Ukraine signed as part of the denuclearization act that says if Russia invades the Ukraine we all could go to war with Russia, but I don't see that materializing either. Europe has been basically quiet so far on their intentions other than disagreement with Putin's actions. It will be a matter for the UN with a possible vote in hopes of embarrassing Russia, but Putin won't care as his desire to control Ukraine outweigh any thwarting.

Putin isn't going anywhere with his government control and might end up looking like a peacekeeper once again.