If another player is harassing you, sending inappropriate content, or otherwise breaking the rules, the way to flag it today is by submitting a support ticket — include their username, roughly when it happened, and as much detail as you can (a screenshot helps a lot if you're able to grab one). Our team reviews these directly.
Separately, if you just want the behavior to stop affecting you right away, you don't need to wait on a ticket response to block them — blocking is instant and immediately stops them from following you or interacting with your profile. It's a good idea to do both: block for your own immediate relief, and submit a ticket so the behavior can actually be looked into and addressed on our end.