Most of the new accounts coming from those members who wants to earn more money with these signature campaigns which is the main cause of spam in this forum.
I think there are a lot of accounts being registered on the forum and being left inactive. You can see many such accounts registered years before without a single post. They are not posting anything here and thus not the cause of spam. May be they are aging the accounts for use in future.
And, you are right about the possible limitations or loopholes for the suggestions I gave.