Patch 9.1: Challenges you can turn off

Friend challenges pop up in the menu and can be turned off. On CrazyGames: Quick Game with every starter deck, and mute support.

Other changes

A friend's challenge now shows as a small window in the main menu, also while the chat is closed: Accept, Decline, or "Turn off challenges".
New setting in your account: "Friends may challenge me to a match". Turned off, nobody can challenge you; you can turn it on again at any time.
Against a flood of challenges: after you decline, the same friend can challenge you again only after 5 minutes, and at most 5 times an hour.
CrazyGames version: Quick Game is open right away with any of the six starter decks, also before the deck quests. Ranked keeps its rules.
CrazyGames version: when CrazyGames mutes the game, music and sounds stay off until CrazyGames turns them on again.