Bring one guy in a jamming ship or have your fleet launch and spread ec-300s. Do anything that increases the multiboxers APM requirement, and watch their efficiency drop sharply.
You are asking for out of game solutions to a perceived problem that already has many counters that you as a player can employ in game to even the playing field. How many players do you think you need to bring to defeat the average 10-account multiboxer fleet, assuming both sides are using a sane fleet composition?