The only thing left would be nuking. He hasn't dared and likely won't.
He has dared. Empty threats? Maybe... but that's quite the gamble to make.
Edit: To downvoters...
Why?
History tells us that appeasement is a worse gamble.
On the contrary: Should Putin risk everything for some sq km of Ukrainian soil?
The only thing left would be nuking. He hasn't dared and likely won't.