This would surprise me given the fairly high karma minimum to downvote.
Make a new account every time you reach the threshold, add it to the bot list.
Yeah, I thought of that, I was just thinking that surely someone able to create sufficient karma to do something like that would necessarily also be unlikely to be petty enough to bother.
Maybe I am too charitable.
You can gain karma fairly casually by just using the site, you get 1 karma per comment, so it's easy to make it a passive activity.
The site has also been around for over a decade, lots of opportunity to accumulate.