Person not on my friends list is messaging me, unable to block him

A person who is not on my friends list anymore started messaging me a week after removing him. I’m not able to block him and no matter how many times I try to report him, it does nothing. His messages only show up on the app and do not show up in-game.

I was told to report the inability to block him as a bug when I 100% should be able to block him through the app.