Putin could end it by withdrawing from Ukraine...indeed never started the war to begin with. There are more NATO members than before because of this and NATO is now more entrenched than ever as a result of these actions and awoke from their slumber as to the stupidity of trusting Russia by tying their energy to them...a bit late for it to not cause serious problems in the meantime, however.
And taking with them the four regions -- Zaporizhzhia, Kherson, Donetsk and Luhanks --- that voted overwhelmingly to become part of Russia, right?