NOTE: I am a moderator over on Ask Ubuntu, and am speaking from my observations and experience there, as well as observations on other sites for moderation, and as part of observing moderation patterns through my efforts with Charcoal and the SmokeDetector bot.
Some of these statements may not apply to this site or meta, however the core principles remain the same. Approach this with that in mind, and please be kind with responses to my thoughts.
The TL;DR: You can't really 'prevent' first time users from posting comments as answers, because they're likely to just breeze through the rules pages without reading so they can post. Do that enough, and watch their answers deleted as a result, and they'll eventually hit a system-placed answer-posting ban at some point, and then they will have to change their act, or never be able to answer again, basically. (Same applies for too many duplicates or bad quality questions, or questions that just get closed as unclear on a regular basis and is a large majority of their posts.) You'd also anger every new user, which would harm the site actively.
Regardless of how you try and restrict this, users will "glance" over the rules so they can get past the system locks, and then post comments as answers either way to try and make their statements known. This is similar to the regular actions taken by users when prompted with EULAs on software installations, insomuch as they agree to the terms but don't read them.
The same thing is applied to posts and rules on SE - even with restrictions as you suggest users will still blow right through them and not read them closely.
That being said, there are automated mechanisms in place where if a user constantly posts comments as answers, and they are flagged or deleted for moderation purposes, the user gets stuck with a answer ban for some time until they prove they can provide quality posts. (Same for horribly poor questions too).
Moderators also, on a site by site, moderator-team-consensus on the basis for this, can reach out to users with mod messages about consistently disobeying content rules (though you probably should NOT wield the Mod Hammer of Doom at the first time you reach out to the user on this kind of case, and just message them first...). (On Ask ubuntu, I occasionally prod users privately when they are consistently being bad, though that usually only happens when they consistently post poor content and also some decent content and do NOT have an existing ask/answer ban).
My two cents on insights into what you propose here on your meta thread. :)
(I have this opinion also: If a user posts a comment as an answer and then flags their post to be converted to a comment, I usually let the user slide. Such users are rare but they are nice heh)
Kevin Reid AG6YO made a very valid point - such restrictions as proposed here are going to anger every new user (with the exception of some users, because some of us get the +100 association bonus so we can post comments), and that'll actively harm the site, rather than help things.
