As soon as a post has been flagged as spam, any links contained in it should be converted to plain text (even if there's a nofollow set). Additionally, when trying to edit such a post users should receive a notice to discourage them from removing the spam content but rather flagging it as spam, too.
The advantages of this would be:
- People not accidentally clicking malicious/spammy links
- Moderators not having to check the post history to see where the spam is