I have inquired with my tech team about the signature spam (FYI this is a constant issue on all forums, not just HN -- the good part is, if you don't go looking at random accounts' signatures that are not even posting, you won't see it at all!).
There may be a way to require members to have a certain post count in order to have a signature. I'll ask them. It may take a while to implement if we can do it at all. And it won't eliminate the existing ones, just prevent new ones.
The only other way to eliminate signature spam is to eliminate signatures. For everyone. And for obvious reasons, we would rather the real members be allowed to keep their real signatures, than ban all signatures just so people who specifically go looking for bad sigs won't see them when they do.
My very sincere advice is, don't go looking at random accounts, looking for sig spam, and you won't see it. Until and unless one of those accounts actually posts something, the only way to see their signature is to go hunting for it. So just, don't. Other than that, it has zero effect on the site, and is very difficult to detect or eradicate.
Thanks all