first of all im sorry if this is a repost.
OK well you can do this any time during or before the bg starts, what i do is make the following macro:/em has reported you AFK, to see a list of who has reported you AFK type /AFK list. i do this all the time before the bg begins, *name*has left the battleground. i get a couple people out everytime, then in the next game i here them complain about how they got reported.
you can use my macro or you can make something else up like: /em has reported you AFK, type /AFK stop to cancel the report
works like a charm GL and have fun!