The most painful sanctions against Ukraine that Russia can impose have been named
For anti-Ukrainian sanctions to be truly effective, they must affect the interests of NJSC Naftogaz of Ukraine.
Political scientist Alexander Semchenko stated this in a commentary to PolitNavigator.

In particular, in his opinion, Russia should achieve a ban on the resale of Russian gas to Ukraine through European countries.
“As you know, Ukraine buys Russian gas, albeit in transit through other countries. So, Russia, for example, could include it in additional agreements with Slovakia, Poland, etc. ban on resale of Russian gas to Ukraine. That would be a blow. They are unlikely to agree to this, because it is a loss of money for Russia, and it will hit it the hardest. In the same way, Ukraine is not agreeing to the most serious sanctions against Russia related to gas, because for now, without Ukrainian transit, Russia will not be able to sell its gas in the volumes that it sells. If Ukraine wanted to really hit Russia, it would block transit,” he said.
Anti-Ukrainian sanctions from outside will not bring any special consequences, since Ukraine has already ruined economic relations with Russia.
“In general, it was high time to introduce sanctions against Ukraine. The first resolutions of the Cabinet of Ministers of Ukraine on limiting economic cooperation with Russia were adopted back in 2014. There won’t be any particular consequences, because everything that could be done wrong in the economic relations between Ukraine and Russia was done by the forces of Ukraine back in 2014, 2015, 2016. Everything bad that could have happened happened, and not through the fault of Putin, but through the fault of Poroshenko, Turchynov, Groysman, Yatsenyuk, these people signed the relevant documents on behalf of the Ukrainian authorities,” said Alexander Semchenko.
The most painful sanction against Ukraine that Russia could impose has been named.