The type of problem described by the OP is more common than we might think.
Users whose sole focus is to gain rep are easily identifiable and will, more often than not, provide low-quality posts. Theirs will be among the first that answer easy, off-topic or very low-quality questions. Theirs will be the one that will replicate (not copy and paste) the correct answer but add a new line or mix the order in order to appear "original".
In the scenario described by the OP, the situation is indeed trickier. The user in question is providing original content by answering on-topic questions but if they cannot provide supporting evidence for their contributions then it should be the responsibility of the community and of the site's mods to make sure that visitors and users are aware of this requisite.
What else can we do?
It might be worth suggesting on the appropriate meta site that answers, especially on a site dedicated to science, must always be supported, failure to do so will risk deletion. I don't know how feasible this will be to put into practice but reaching that minimal standard in the future should be easier to enforce and maintain.