Just delete or ignore the message. There's also "Prevent others from sending you friend requests" if you want the nuclear option.
When you "friend" someone it's more like you're "following" them. It's completely unilateral, but also doesn't really give you all that much. You can "stalk" people's profiles easier and invite them to private Shiritori games, but that's about it.
Honestly, the only reason I even have friend requests on is because I want more Shiritori games :D
