What is the best way for an OP to announce they will be strict with answers, meaning answers that don't meet the conditions and terms of the question will be marked low-quality and deleted?
This should be our normal way of doing things. If you ask how to boil eggs, it's not helpful to you if I post an answer on how to make an omelet. It doesn't matter how great the omelet-making answer is, it still isn't an answer to a question about boiling eggs. You shouldn't need to point out in the question that you aren't interested in answers on how to make omelets, or apple pie. That should come by default.
Heck, if you ask how to boil eggs, unless you allow for magic answers, it's not even particularly helpful to you if I post an answer describing how to wave your magic wand for the raise-temperature-of-object spell, or even suggest that you might have a spell to raise the temperature of an object that you could just apply to the eggs in question.
Frame challenges is more about telling OP that they really don't want to boil all of the eggs if they want to keep their poultry over time, and so if they want an industrial-scale egg-boiling machine, even if they're asking how to boil as many eggs as possible in as short a time as possible, they really shouldn't feed all of the eggs to it.
Is a tag (e.g., strict-answers) sufficient, or should the OP use a custom announcement in the question?
Given the above, this question more or less comes down to "do we need a tag or special disclaimer to indicate that we should do what we're supposed to already be doing?".
I imagine that what I feel should be the answer to that is obvious from the phrasing.
Meta tags are problematic, and have been discouraged network-wide since 2010.
Just here on Worldbuilding Meta, there's Is “science-based” a meta- or otherwise-problematic tag? and Do we really need an anatomically-correct tag? and Is the tag [logical-consistency] workable? and Why no “big-list” tag? and Mutually-exclusive goals and how to discover them and probably a slew of others that a quick search didn't uncover.
Robert Cartaino's answer to Are meta tags banned across the board on all Stack Exchange sites or can each site decide if it wants to allow meta tags? on Meta Stack Exchange has some more background.
On Meta Stack Exchange alone, there are 93 questions discussing meta tags directly and by term, and there's probably many more where they are discussed by that name only in answers.
We already have several meta tags on Worldbuilding, which to top things off are often used incorrectly. I think it's been a while since I saw a question tagged only, say, science-based, but that used to be a regular occurence. Maybe the community is just quicker at editing the tags on those questions these days. These tags don't tell us anything about the subject matter of a question; they impose restrictions on answers, in a way not obvious to a newcomer (whether that newcomer wants to post a question or an answer). Adding more such tags seem to me to be a really bad idea.
Rather, unless OP states they are okay with alternative solutions, judge and answer the question based on what they are actually asking. If the question cannot be reasonably answered as asked, put on hold and get it fixed, whether that requires changing the premise or simply clarification of the purpose for which the question is being asked.
To top all of this off, I distinctly recall that at one point there was a proposal for a tag that would allow for frame challenges; basically the exact opposite of what you're proposing here. I can't seem to find it at the moment, and as I recall it got shot down pretty quickly, but if someone happens to come across it, please comment. It's out there somewhere...