Online matches have a text chat. You can send to everyone, or to your team only, using the toggle beside the input.
Chat is relayed between players and is not moderated in real time. Nobody reads it as it happens, which is why the two controls below exist.
Every other human player in the match gets a Mute button above the chat bar. Muting stops their messages reaching you for the rest of the session — you will not see them arrive, and nothing of theirs is kept.
The mute lasts for the match and the lobby it came from. It does not follow a player into a future match.
Report sends the moderators that player's recent chat along with your complaint, and mutes them at the same time so you are not left reading someone you have just reported.
Reports are read by a person. Bad-faith reporting is visible to whoever reads them, so use it for behaviour rather than for losing.
Neither control appears for computer-controlled raifus. There is nobody behind them to mute.