We already do it with drug cartel bosses money and this pussy-footing around because it might make our own billionaires nervous is playing exactly into their hands.
Sure, the money that is sitting in piles on the floor, hidden in walls of highly fortified compounds or in duffel bags in vans full of drugs or guns that get pulled over and the origin of which cant be better explained or proven. It's a bit different to taking something away which appear to have legitimate origins or have enough reasonable doubt as to not be able to be definitively proven otherwise.
Its worth noting that the cartels also participate in laundering money through the various international loopholes into legitimate enterprises which makes much of those assets untouchable. There are profesionals in this field that spend decades unraveling this stuff. It will however in the future be easier with stuff like blockchain making its way into financial markets bank ledgers etc.
Many of the worlds legitimate wealthiest people have various legitimate arrangements that are in these grey area. Those people have influence. there wont be any wholesale world wide changes that impact those people just so we can get one up on Russian oligarchs and kleptocrats. Its wishful thinking. We'd have done it by now if it was that simple.
If we do fuck all now we WILL be at war. Putin will not stop with Ukraine and doing nothing will only embolden other countries with territorial ambitions - namely China. At this stage military measures are not an option but sitting with one hand clutching our pearls while the other has it's thumb up our arse is not an option either.
Maybe. Maybe not. I mean They invaded Crimea. No one did anything. Georgia. No one really did anything. They blew a plane out of the sky with international passengers on it. No one did anything. They routinley poison people with nerve agents and radioactive isotopes to send messages, even in the UK. No one does anything. China has been abusing millions of people, claiming territories, moving into tibet and other associated territories. No one does anything. Bosnian war resulted in nothing of note on a world scale. Afghan and Iraq wars didn't result in anything really. Vietnam war etc. There are conflicts all over the world all the time that are outrageous but haven't resulted in a world war. I don't expect this to result in anything significant unless Russia starts invading more countries or uses nuclear weapons (the latter is highly unlikely).
Even if asset seizure is in the too hard basket (which it isn't for a lot of things - superyachts for example. - they need harbours to be moored in, have to declared seaworthy by authorities etc. ), declaring individuals as persona non grata is something that can be done quite easily. Visas are still a necessity for travel and they can be revoked at any time - just ask Novak Djokovic, Katie Hopkins etc.
Asset seizures are not simple. Trust me. I had to arrange the seizure of Nathan Tinkler's gulfstream from Singapore when he failed to make payments on it. In a world with all the legal documentation in order and laws in BOTH countries in agreement and in our favour, it was not a simple process and took the better part of a year to recover.
As to Visas - not all countries care for these matters. The largest western democracies are pretty tight on this stuff, sure, but its not the be all and end all. People get around this stuff routinley. The more money you have the easier it is obviously. again if it was simple, it would have been dealt with already. There is even evidence that some of the 9/11 hijackers entered the US on doctored passports.
About 10 years ago a ex arms dealer from the bosnian conflict was caught in Melbourne under an assumed name. He'd been living here for a really long time, had a family, paid his taxes, had a tax file, got into the country on a false passport and visa. NO ONE picked up who he was until he applied for a protective services officer role and the police did an extensive background on him. The guy felt safe enough to try that on. What does that tell you about the ease of evading sanctions and watchlists? We found out about it the following day when his assumed name came up on the daily watchlist and adverse media screening against our databases.
Putin's allies' lavish lifestyles are almost entirely lived out in the West. We can put a stop to that shit easily. It might not make much of a dent in their fortunes but what's the point of being a billionaire if all the fun things that billionaires do is locked out to you? That would be enough to get them to start applying pressure on Putin.
Being an ally or friends with Putin is not a crime. Unless there is a crime that can be definitively proven in the jurisdiction that is to do the seizing or there is an international co-ordinated agreement to seize (unlikely), not much can be done against these individuals. Sanctions are basically the only thing that is able to be imposed. But i can guarantee you that the majority of these peoples assets wouldn't be in the name of the individuals or the companies they are associated with that are making it to the watchlists - ergo, hardly effective. Some countries will not allow these people in (under their name) but how many countries are ripe for corruption? You think a Russian oligarch doesn't have multiple identities or a legitimate stack of Venezuelan passports to travel on? Does sitting on a south American beach really differ that much from one in the US? The world is a big place and you go where you can go that wont cause you troubles, and to the ultra wealthy, thats more places than you think.