Hegira

If they're commenting to your locked post, which is where most of the concern is, you do have them friended. No, it won't solve crossposting from your public entries, but it's more likely to appeal to LJ's owners than turning off the "feature" or allowing journal owners to disable it, given that there's probably a purely financial/business motivation on their part. We already have the coding in place where your recent comments show up on your home page; making a crossposted comment show up in your journal shouldn't be much more complicated than that.