This AFK issue was brought to Dan's attention in the old Fleet Marks thread. I am sure he was aware of it before then. At this point they have not given any indication that reporting an AFKer will have any consequence to that player. I'm not sure if it's even worth reporting.
Dan said they are aware of the problem and are looking into options to combat this.
In another game I have played, team members can report a player AFK and they will be marked as idle, preventing them from receiving any benefits from the run until they engage in combat. Sure, this may not work with trolls looking to just run around and cause problems for your group, but to an AFKer this is probably the worst thing. If we had something like that, I am sure it would significantly reduce the AFK problem once people were aware that they wouldn't get anything if they just sit in a run doing nothing.
Personally, I rather boot them out of the event and hope a replacement is allowed in.
Because in that flag system, an AFKer isn't still going to help you. You are stuck doing the event with four people. While some groups can easily do events without a fifth, many groups cannot. And those four players are very much likely to suffer no matter what, they will fail the Objectives (or the STF if it's Khitomer or Cure Space). Which easily turns that flag to into a self-inflicted griefing tool.
To put it bluntly: Oh you can flag them, but it isn't going to help you any.
So if Stahl is going to implement a Moocher Flag, he better have that flag adjust the event for the other four people.