I know this isn't the kind of response you are looking for but you don't need to let it clog your blacklist. At most there will be 3 active accounts spamming (usually it's just 1 or 2 on my server). Clear out your list entirely and ban the handful that come in. I delete/clear out my blacklist about once or twice a week. Is it ideal? no. But I find it fairly easy to manage so for me it's a pretty mute issue.