It’s possible. I think it’s more difficult than people think. You have to do it on a scale which is blatantly obvious to anyone who’s looking, so you’re just inviting a ban.

One person swore to me that it would be really easy, so I invited them to try, and they made a gang of bots which farmed karma and then mass-downvoted me, trying to get me banned from my own place. If you look at my profile you’ll see some things which have -300 score because of it. I welcomed the effort, since I’m interested in how well it will resist that kind of attack. Their first effort did exactly nothing, because none of the downvote bots had any rank within the algorithm. I gave them some pointers on how they could improve for a second time around, and they went radio silent and I haven’t heard from them since then.