Had an inter, client crashed after game and couldn't report him, how else can I report him?

Take a screenshot of the match in your history, then open a support ticket and explain the situation with the file as an attachment. Riot will redirect the ticket to the correct people, and you'll receive some copypasta response thanking you for your report, telling you that it's better to use the report feature, and that Riot cannot disclose any action that may or may not be taken against the player's account.