Putin should threaten to drop a tactical nuke on Kyiv in 7 days if Ukraine doesn't surrender. If they still refuse to negotiate after Kyiv is nuked then he can threaten to nuke Lviv next. This would give civilians enough time to leave the city.

The US nuked Japan twice to end a war, and that was without any warning. If Putin gave a week's notice that would be enough time for everyone to leave the city to minimize civilian deaths.