Perhaps then have a system where admins can ban an account and every account that branches from their invite. For an obvious spam account you can ban it, ban every other spam account they invited, and investigate whoever invited the spammer originally. That way you cut out the root of the spammer tree and all the branching spammer inviting spammer inviting spammer dozens of layers in falls immediately. Admins could use this tool at their discretion, so a person who has previously participated in good faith but now needs to be banned doesnt have to bring down all the other good faith users they brought in.
Perhaps then have a system where admins can ban an account and every account that branches from their invite. For an obvious spam account you can ban it, ban every other spam account they invited, and investigate whoever invited the spammer originally. That way you cut out the root of the spammer tree and all the branching spammer inviting spammer inviting spammer dozens of layers in falls immediately. Admins could use this tool at their discretion, so a person who has previously participated in good faith but now needs to be banned doesnt have to bring down all the other good faith users they brought in.