I recently saw a spam question, which i then flagged as spam and posted this an answer (and deleted it very quickly after, but you could still see the post):
ADVERTISING IS NOT ALLOWED HERE, GET OUT
Can we do this on a spam question?
Why do you assume it is posted by an actual person?
It is more likely an automatically generated message.
I know it's policy not to edit spam questions. I would think posting that sort of an answer would just be egging them on and wasting your time. It's probably posted by a script and not a human anyway.
Spammers already know they are crossing the line, so no new information is conveyed.
Worse yet, answering their post bumps it and gives it more views. Also, assuming they are monitoring their spam, they may conclude that they've at least gotten the attention they want.
There is one case in which you could respond to spam: if it is disguised, and not immediately obvious that it is spam. In that case, after flagging it as spam, leave a comment under the post warning future reviewers and moderators.
Another thing to do with spam, if it isn't being removed quickly enough, is to bring the issue to the "Tavern on the Meta" chatroom. In Tavern on the Meta there is a bot, called Smoke Detector (affectionately called Smokey) that reports spam and maintains a blacklist of known spammer accounts. The developers have given some regulars the privilege to give the "!!/report" command to Smokey, which causes it to report the spam post in several chatrooms and blacklist the spammer. Here "blacklisting" means that future posts from that account will be automatically reported by Smokey as well.
Even if you're not one of the users allowed to give commands to Smokey, most regulars over there are, and will be glad to help.
In practice, spam is removed from AU amazingly fast, so you're not likely to need to visit the Tavern chatroom.
You risk your own messages getting flagged as spam, and justifiably deleted along with the spam which may result in you getting blocked for spam as well.
Even though you may have good intentions, you would be posting content that is -
Thus I would flag any sort of these attempts as spam as well, or at least not real answers.