Next time you fight against a multiboxer in WSG, get one person to take the flag, and actively stay away out of the multiboxer's way. The multiboxer has then two things to do: Either pursue the flag holder or go for the flag themselves.
If they go for you, that means that most of the team is in one place on the map pursuing you. All the flag holder does now is run around and avoid; they will inevitably get you, but it will be easy to get the flag again as there are likely no players in the flag room.
If they go for the flag, your team needs to go for their flag (if you haven't already) and give them plenty of space to cap it. If they go for the flag holder, pop defensive CDs, sacrifice some team members for CCs and run away. If they go for their base, you've pretty much got them.
Form up a large group of 6+ players. Make sure that at least one person is defending your flag holder, and go for the multiboxer. You should have it in the bag, that's if you guys don't go in one by one, or get trolled by starfall. A psychic scream, thunderstorm, whatever CC's you can land will land this in the bag.