You might point them at the help page of vote-down privilege which explains a bit:
When should I vote down?
Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.
You have a limited number of votes per day, and answer down-votes cost you a tiny bit of reputation on top of that; use them wisely.
There really isn't any better resource that I've found.
Personally I always like to point out that nobody complains about getting an up-vote and gaining some fake internet points without a extensive explanation.
I use the AutoReviewComments action script with the following pro-forma text to explain downvotes on a Question:
By philosophy and design votes are anonymous and neither voting up nor voting down requires any mandatory explanation. The tooltip that appears when your mouse pointer hoovers over the down button states: "this question does not show any research effort; it is unclear or not useful". Also questions can attract a down vote when not well written, not quite on-topic or missing details.
I rarely use it but I also made a slightly different stock answer to explain downvotes on Answers:
By philosophy and design votes are anonymous and neither voting up nor voting down requires any mandatory explanation. The tooltip that appears when your mouse pointer hoovers over the down button next to answers states: "this answer is not useful" which may be subjective. The instruction is to vote down on any egregiously sloppy, no-effort-expended answer or one that is clearly and perhaps dangerously wrong. We also don't like spam or answers that only contain links/code/commands.