How do I report a player that hold champ select hostage?

I just had a game where I was in lobby with a guy that intentionally banned a otp lux players lux. He dodged. Next game we got in the same lobby, he bans our adcs kalista on purpose. Last 10 games he have lost 9/10 games with smite ghost jg. Since I dodged the second lobby I can't report him at the end screen so how do I report him now? What can I do?
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