Definitely not an easy one, and in the absence of any suggestions of my own, I feel a bit mean in criticising yours. Oh, well...
You are spot on in identifying him as a gangster: hitting his mates financially is absolutely the right thing to do, unfortunately we've been doing that since not long after he took power, and there aren't that many more easy targets. All the obvious sources have been tapped already. We could probably put a bit more of a squeeze on, but there is a cost to doing this sort of thing. God knows what nefarious deals have been made with his buddies by our own elites - there's possibly too much to risk for the sake of a couple of Russians, a policeman and a minor chemical attack in a minor English town. Flippancy intentional - but that's the way our decision makers will have to look at it.
The "making Russia great again" thing is true. A bit of agro with the west does him no harm politically. No amount of sanctions can change that.
The Iran thing is simply wrong. Russia has a very temporary, very shakey "alliance" with Iran over Syria only. The Iranians hate the Russians nearly as much as they hate the Americans and Brits. Nothing to be gained there (as far as hurting Putin is concerned) - if anything, he'd welcome any souring of relations between UK and Iran.
Syria - well, we've well and truly missed the boat on that one. There was a time (circa. first vote on intervention), before Putin committed to supporting Assad, when this could have worked. Though, equally, intervening could have gone horribly wrong (and based on our other recent adventures in Muslim countries, almost certainly would have done). Won't work now - he's thoroughly committed, and UK has neither the means nor appetite to play in that theatre.
Cyber aggression - I think we have much more to lose, tbh. We could hurt Russia, Russia could hurt us. Putin would still be there, posing topless on a horse for his official 2019 calendar.
As I say, I have no better suggestions. He shouldn't be allowed to send bombers into our airspace, send submarines off our coastline, invade friendly countries, annex bits of Europe, support genocidal regimes, disrupt our democratic elections, murder journalists, defraud his population, carry out assassinations in the UK using radiological and chemical weapons, etc. etc. But what can we do? Vlad will be Vlad.