For starters, Obama could use the threat of freezing assets/seizing Russian accounts in the USA. Remind them we have a couple of ships in the Black Sea.
Russia does have interests in the Crimean Peninsula, however. But we need to let them know we’re watching and actions need not escalate. Just a few friendly reminder.
And one of those ships is out of commission due to having run aground and damaging it's propeller.
The other ship is an amphibious command ship. How much damage do you think that one ship (or both, if the other one were in service) could do in the face of the Russian Army...or Navy?
You don't suppose Vlad has tasked a sub to keep an eye on them, do you? I'd be willing to bet he has.
Step out of line...fire one, fire two, and two ships go to Davy Jones' Locker.
Don't think he wouldn't authorize it.