The best course of action would be to let him have Eastern Ukraine and the Crimea; the majority want to be Russian anyway
Be very, very wary about believing anything that's been reported on this subject. Putin knows full well that the west's greatest weakness is public opinion, and the best way to target public opinion is through the media. That's a hell of a lot easier today than it ever used to be; even the mainstream news outlets like the BBC have lowered their editorial standards to avoid being scooped by social media, and will gladly broadcast most things that are sent their way. Crimea wants to be Russian? Youtube video of some people waving Russian flags? Get it on telly, quick, before ITV! Now, as far as most of the voting UK public is concerned, it's fact - if they're inquisitive, they'll look it up on Wikipedia, which has almost certainly been doctored by the Russians as well - and so there is absolutely no public appetite for a confrontation. Game, set and match to Putin. Once upon a time, our media would have resisted outside attempts to use it as a propaganda tool. Now, it's just too busy with its 24hr rolling news cycle to care.
A week later, some international relations expert will publish a detailed article on the ethnic background of Crimea. By then no-one will care.