He is Jewish, and an Israeli citizen. Just because he was born in Southern Russia, and made megabucks under Yeltsin, I’m not sure why he needs to be banned. George Soros was a Nazi synpathizer, and once wrecked the British pound, yet they don’t ban him.
Your very solid question leads to a very uncomfortable answer.
Russian oligarchs, many of whom made their billions under a kleptocracy, stand to lose everything under the sanctions. No more villas in Tuscany. No more mansions on the French Riviera. No more artwork. No more mega yachts. No more football teams. Everything.
They're being targeted with the belief that these are the only people who might be able to restrain Putin.
I don’t think he should be banned, but I think you are underselling it. Abramaovich was the one who told Yeltsin to make Putin his successor. Without him, Putin might be a former KGB that you never heard of.
Also, Abramovich had approval rights for Putin’s early cabinets, and he picked Medvedev. His biographer said he is Putin’s father figure.
It's inaccurate to say he was a Nazi sympathizer. Maybe a collaborationist. He delivered slips of paper.
He didn't "wreck the British Pound," he blackmailed the British government into doing what he wanted, by threatening to devalue the Pound.