Reporting players outside of the end screen?

How can I report someone who was just being toxic in the lobby? We were trying to start a game using dynamic queue. This guy get's jungle but let's everyone know immediately that he's going top. I tell him he got jungle so he replies im going top. We threaten the report and he starts slinging profanities at all of us until someone else in the lobby just dodged so I never got the chance to report. I have screen shots but I can't post them on the board?