Have you tried going around the multiboxers or avoiding them? You can apply counter zerg tactics to make the multiboxer's life harder (or more boring).
Biggest drawback to multiboxing is they're stuck in one place and have to move together.
People who complain about multiboxers are like people who complain about ending up in the hospital after playing in the traffic.
If you know you're about to run into a meat grinder, turn around? Go around? Bring friends? Fear bomb? Knock backs? Hello? Is there a brain up there? Don't just willingly walk / run into your death then complain about getting killed.