Crimea is largely useless if the US and other noble powers blockade the dardenelles. (and yes a blockade is an act of war)
Actually they could just sit in Ukrainian waters and blockade any part of the Ukraine they like legally with Ukrainians permission.
Do a "feel free to leave, but you can't come back" sort of thing. There lots of all sorts of technical things they could that could stumble the Russians unless the Russians wanted to go to war.
The fact is we don't have a spine in the west, even though there are technical measures that we could do that could allow him a way out if he wasn't prepared to escalate.