There is a way you can enter a description of what has happened, but not through the report player option. Open up the menu and you have the support selection. There you can submit a bug report and you have options in drop down menus there to select what part of the game and also to enter a description.
I only use this to report players who are using hacks in WvW (teleport or speed or jump hacks) I select the player and do a normal report on them and also take a screen shot/s of him best I can using the hack and attach them to the report (there is an option, as well as an auto-screenshot attachment option) and enter my description of what happend / what the guy was using and also mention I did a normal player report at such and such time so hopefully they can look that up and clearly identify who it was. Of course since bans aren't discussed with anyone outside the person being banned no way to know if this method works or not, but its certainly better than posting it in a forum.