How do I block an Xbox player from sending me game requests and seeing me online

@Splashfart213Oldandknackered is correct you can find this multiple ways, 1 is to search for them.

press xbox button on your controller go over to the people icon/menu. Down at the bottom you'll see find someone.

If you can't find their account that way, just accept the invite and join them for a moment. When in the lobby or party bring up their xbox profile. You will see the ability to block them.

You might still get matched with them randomly in game. But they can not invite you or communicate with you. I got matched with someone I have blocked it was really odd. The really weird part? We won the round neither of us carried the other.

If you're still having trouble with it let me know. I'll join you in game and walk you through it.