Just boot them. I've never seen an AFK vote kick not go through.

We had this same issue when tomestones were first added to Frontline, and unfortunately we're going to see more of it now with PVE EXP rewards. I definitely think a few problematic AFKers is worth it though. For every 1 person I've seen joining Frontlines to AFK for free EXP, I've heard 5 players who have never done PVP before join up and say they love it. Despite a couple hiccups, Frontlines has never been in a better place than it is now, and I think SE did an incredible job revitalizing it.