Proof of work challenges are quite effective against bot swarms. Some data of my experiments:

You may know about Anubis by Techaro, the PoW challenge thing that protects websites from bots. It's used on several major sites, including FFmpeg, Arch, and the Linux Foundation. This experiment is specifically about Anubis.

Note that Anubis does not use up all CPU cores for its challenge to not overheat devices and for a better UX. Some PoW challenge systems do all cores, making them more effective. However, it appears as if Anubis gets the job done just fine.