I just rejected a spam edit on a post today and out of curiosity decided to check the history of that post. It turns out that same post has been the target of repeated spam edit attempts in the past. Looking back at some other rejected edits in the history for other posts I see similar things have happened too. Some posts are the target for repeated spam edit attempts.
Thankfully all these edits were reviewed and rejected, but it strikes me as something that would be useful for moderators to know about. Perhaps a flag stating "{this post} has been repeatedly targeted for spam edits".
However this also brings up the issue of what a Mod can do about it. Aside from protecting the whole question (not ideal) or deleting the answer (again, not ideal) there's very little we can do. I think that by editing the answer so that it is longer and more substantial reduces the risk of it being targetted again, but that might not be appropriate.
So I suggest the following (and am open to alternative ideas):
If a post has had 2 or more different suggested edits reviewed and rejected as spam then it should automatically raise a flag and be blocked from being edited by <50 rep users.
Possibly the option to Protect the post itself could be applied, but I'm not 100% sure this is ideal because I don't want to prevent people from genuinely editing posts. Therefore I'm open to suggestions as to what actions a mod should be able to do on such posts.