I don't know anything about LW's internals, but in most sane architectures you'd want to have a last-downvote timestamp linked to each user's account, and to compare the timing of incoming downvotes against it. If the difference is too small, you reject the input and display a message similar to the one you get when you're about to post in a heavily downvoted thread. That message has a few issues -- it doesn't come up if the vote total upthread dropped below the threshold between when the page was loaded and when the post was submitted -- but since we only care about session-local behavior here, that shouldn't be a problem. Synchronization between the session and server time might be harder but that depends on details I don't know.
I can see a behavioral pitfall, though. I typically decide on votes after reading through a screen's worth or so of a thread, and this sometimes implies two or more downvotes (for example in the cases of short low-quality posts or discussions inappropriate for the site). I'd find it gratingly aversive to be forced to wait N seconds or minutes between posts in these cases; I don't know if this happens often enough to outweigh the mass-downvoting problem, though.
To whoever has for the last several days been downvoting ~10 of my old comments per day:
It is possible that your intention is to discourage me from commenting on Less Wrong.
The actual effect is the reverse. My comments still end up positive on average, and I am therefore motivated to post more of them in order to compensate for the steady karma drain you are causing.
If you are mass-downvoting other people, the effect on some of them is probably the same.
To the LW admins, if any are reading:
Look, can we really not do anything about this behaviour? It's childish and stupid, and it makes the karma system less useful (e.g., for comment-sorting), and it gives bad actors a disproportionate influence on Less Wrong. It seems like there are lots of obvious things that would go some way towards helping, many of which have been discussed in past threads about this.
Failing that, can we at least agree that it's bad behaviour and that it would be good in principle to stop it or make it more visible and/or inconvenient?
Failing that, can we at least have an official statement from an LW administrator that mass-downvoting is not considered an undesirable behaviour here? I really hope this isn't the opinion of the LW admins, but as the topic has been discussed from time to time with never any admin response I've been thinking it increasingly likely that it is. If so, let's at least be honest about it.
To anyone else reading this:
If you should happen to notice that a sizeable fraction of my comments are at -1, this is probably why. (Though of course I may just have posted a bunch of silly things. I expect it happens from time to time.)
My apologies for cluttering up Discussion with this. (But not very many apologies; this sort of mass-downvoting seems to me to be one of the more toxic phenomena on Less Wrong, and I retain some small hope that eventually something may be done about it.)