One way Ukraine could tangibly hurt Russia in the short-term would be to shut down the gas pipelines on their territory. Consider this map:
No doubt this would annoy the Europeans who receive Russian gas. But they’re not actively helping Ukraine just now. The Russians would be terribly angry, but they’ve already invaded.
For a country with its back against the wall, I see little downside to shutting down 66% of their invader’s cash-cow