Even if it had been categorically stated that Ukraine was not going to join NATO, I doubt that would have stopped Putin. He’s bent on re-annexing the former Soviet lands.
Effective sanctions would IMO have to hit rich and powerful Russians hard, e.g. by confiscating their assets, at least until this megalomaniac thug is got rid of, so that pressure to oust him would come from within.
IMO we should also have a ban on Russians entering the U.K. because I can see Putin reacting to sanctions with more Salisbury-type episodes.
Or at least have minute inspections of anything they’re carrying, and make them taste anything that could conceivably contain the poison.
I dare say that should also apply to anyone with passports from former Soviet states, since he’s surely bound to have henchmen based in those, too.