Someone harasses me in the post-game lobby. What do I do?

Report them, then take a screenshot and file a Support Ticket about them. Don't listen to the people who say that you can't be punished for post-game chat harassment as you most definitely can.

if u block them they cant chat to u pregame post game or during game

You sound hurt about something someone told you online hehe xd

Maybe ignore it at the lobby and just mute them in-game

Mute him when game starts, then report him after the game. Add in the comment box, "Toxic In Post Game Chat" One report is all that is needed to flag a ticket. Some take longer than others though.

Hope this is helpful, Your Everyday Otaku San [slayer-jinx-catface]

report them pony boy.

report verbal queue for a new match

Harasses you? Wtf dude just leave the fucking lobby.

Screenshot it, go to Riot support and send a ticket with the screenshot.

That's all you can do. Odds are Riot wont do anything on it though. It's too much effort for nothing to happen when you do it.