Well, as not particular example is being discussed, I'm providing generic answer:
First of all, one-liner answers are discouraged. On the Stack Exchange, we emphasize on quality answers. So, whenever we see one-liner answer, we should put comment linking How do I write a good answer? If such answers are upvoted and if question reached HNQ, this would be problematicr1 as visitors get wrong idea that one-liner answers are well accepted (users aren't taking time & effort to write good answers). So, we should not upvote them regardless answer is correct or not.
Now, talking about flagging, You can flag them as VLQ but as pointed out VLQ flag may not be available if answer is upvoted or old (This again indicates that such answers shouldn't be upvoted.) To discourage we can downvote them and also cast delete vote if it is feasible.
Worth to note that here mostly such answers would be unsourced. As titled " flagging one/two liner answers which are without any sources?" Considering the policy of citing sources, you can always flag them with custom reason saying no sources/references are cited and requires to put "Citation needed" post notice.
Note that as I mentioned in comment moderators usually don't take an action on VLQ flags since that is mostly handled by community through review queue. Otherwise you can flag them with custom reason indicating the problem that is answer sounds like comment (you may use flag Not and Answer for that) or the post doesn't provide sufficient explanation or unsourced.
We've couple of suitable post notice which will be added by moderators:
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.
Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.
By the way, if your flags keep pending and you feel urgent action is required, you can ping us on moderation room.
Special/exception case What to do with those type of questions whose answers are simple yes or no?
r1 if one-liner answer is working then there should be problem with question. We expect question should be clear and detailed so-that it gets quality answers.
Ref: Should we be more strict about one-line answers?