There are two different ways that you can report a room.
When reporting from a room
Tap on the room description tab in the top right of the game screen.
Reporting from Published rooms list
Select room you want to report.
Then follow these next steps:
Tap on the red flag that appears.
Tap report.
Select if the room name or description is inappropriate.
Tap next and then select what terms of service the room violates.
Tap send report.