Well, you can not report ingame.
Couple of months ago, there were a toxic player sending offensive letters to people in my server, so I gathered some letters, took screenshot and sent to tibia support email (support@tibia.com). The "CM" or something said screenshots aren't proof of that, and he asked me to send the letter to him by parcel, so he could check by himself. I think they should implement a letter report because not all CM's would do that in my case.