We've had similar problems before with comments saying just use magic
This is basically a duplicate of a problem that has already been discussed on this site:
Are comments saying “just use magic” problematic?
In the linked question the community talked about comments saying just use magic. In this question we should discuss answers saying it's fiction and therefore you can do whatever you want. Comments are only for clarification and can be deleted any time for any reason, which makes them less important than the answers for which this site was built and that can and should be curated by the community. That's why I feel it's important to have this discussion again in this slightly different context.
Everybody knows that they can write whatever they want - that's not helpful
The reason WorldBuilding exists is to ask questions where a writer of any kind thinks what he is doing might break the suspension of disbelief of his audience. Every author is aware that he can do whatever he wants. Mentioning this obvious fact is not even slightly helpful.
Flag it and delete it - write whatever you want is "not an answer"
Furthermore we are not a discussion forum. An answer is supposed to answer the question. It's okay to make an Addendum at the end of your post and mention your concern that the OP is going a bit overboard. But in itself do whatever you want is probably never going to answer a valid question. If you truly feel that this is the right answer to the question you should probably flag the question as "primarily opinion-based".
If the question is a valid question and an answer only says that the author can do whatever the wants without being an useful answer towards the question asked it should be flagged as "not an answer" and consequently deleted.
Start from the point the author set and answer his question
To cite and paraphrase Monica in the question I linked above:
Yes, sometimes you do need to quit your job or handwave your magic, but that's for the asker to decide. We should accept the parameters of the question and go from there.
Yes, sometimes you do need to ignore the science and write something that is not correct, but interesting for the reader, but that's for the asker to decide. We should accept that the asker feels it's important to get something right or believable and go from there.
Sometimes it's okay to remind people that most other people won't take everything in their story apart as long as there are no really big problems. But that is worth a comment at most and only in rare cases. Most of the time I would probably flag them as "no longer needed" if the author didn't make some helpful hints about the specific topic and directed at the question of the querent.
And the whole problem is also similar to When is “that's not possible” an appropriate answer?. Again, on its own that is not an acceptable answer and not an acceptable comment. The writer has to explain why that's not possible, you have to use magic and write whatever you want are the correct answer in each context, tailored towards the problems the OP is facing.
If the writer is explaining why write whatever you want or similar phrases are correct their answer may be valid if they devote more than just a sentence to explaining the why and tailoring their answer to the specific needs of the querent.
Otherwise: downvote, flag, comment why (for example by linking to this answer) and move on.