It is nothing unusual, nor criminal, for a country to enroll foreign
citizens as soldiers. The US has done this in recent decades, and
France's foreign legion is well known.
I don't see how his attempt could be thwarted except by capturing him
and putting him on trial — but that seems awfully unlikely.
One idea for retaliation is that the UK could announce an undeclared
war with the supposedly independent Donetsk regime — not with Russia!
— and start bombarding its supposed "territory" with conventional
weapons. If that hits some of Putin's soldiers from Russia, he can
hardly complain.
The mayor said that "Initially, the occupiers involved Mariupol
residents in dismantling the rubble carefully." But when they saw
that there were 50 - 100 corpses in each building, they took the
residents away, demolished the buildings quickly, and carried
the rubble off with the corpses.
No one will ever be sure who died, or even how many. The most
we can say is that it was probably between 60,000 to 120,000.