How can I report a player on the app?

Send an email to support@chess.com.

https://support.chess.com/customer/portal/emails/new is the link for more info.

Yes.  this post will change many lives and ways for the greater good.

I've never actually played chess with any of my nubile young masseuses, but if one of them started talking dirty to me during a massage, I would just talk dirty right back, and I have a whole lot more experience than she would. 

Same here, I recieved this one by [removed,  public accusations not allowed -- VP], after I won of course (not even on time or by mean method) :
"

Today I received two messages from a player:

[removed image, public accusations not allowed -- VP]

I'm sorry you have to put up with that. However, you might want to censor that post.  (The names part- you are accusing your opponent- not allowed.) 

You are supposed to write an email to the staff (use form link above) and they will deal with the situation.

Many people think we aren't allowed to name the "suspect" (!), well as long as it's only suspicion, yes that's true. We aren't allowed to accuse someone and name him publicly without proof.
But when it's just a proved fact, I think we can ! Don't exchange the roles  If he doesn't want his insults to be be publicly exposed, then he can just NO insult people. 

But yeah, I agree, these are quite common and we'd better not be touched by this stupid behaviour, else we could as well keep ourselves at home and talk to nobody, because that's a cruel world ^^'

ZygimantasJ, if you really want to answer, then you can say "OK I lost on the board, but I won outside, judging by your mental weakness (it seems you have a superiority complex, meaning you desperatly need more confidence, and that explains why you're incredibly happy to have won).

"^^^

The site flags players that do that a lot and restricts their accounts.

https://support.chess.com/customer/en/portal/articles/1444922-fair-play-policy

@Fred_Huspor, that isn't foolproof. It is trivially easy to modify the client side and make it look like someone said something they did not, then screenshot it and post it. 

For private messages, the best is to repirt via the link, leave the messages, and staff can look at them. For chats, screenshot and provide that when asked, but they will look at other things as well.

 @Martin_Stahl
What exactly isn't foolproof ? You interpreted your way, sorry but I never said that showing a message here is proof.
I meant that we can't accuse someone of something that isn't proved, but we can if there is proof.
And the proof is not the quote - I agree with you on that - it is his message to me, in my box, to which, as you said later, the chess.com staff can access.

Simply said, if you have no proof, then you can be suited for false accusation, but if you have proof, then no. Because it's not a "false" accusation.
And the target of these accusations knows well that I do have proof, as he committed it himself

That's what I think but if someone knows how the law precisely works, please tell us ^^

This said, here it's no name shown on public place, it's only aliases, so maybe it's not even suitable. 

It doesn't matter if one clains to have proof though. Screenshots can be faked and anything can be posted in the forums. That is one reason why posting accusations in the forums about it isn't allowed and to open a ticket to support.

You should read again what I wrote ^^'

I read it. The site doesn't allow public accusations, with or without merit.

OK you mean the site, not the law ! Thus explained, i agree then