When you set up a Cloudflare account, you tell your domain provider (Namecheap, Godaddy, Google etc.) that you want Cloudflare to have complete control over your domain by setting the domain's nameservers to Cloudflare's.
If you want to get off of Cloudflare, you just go to your domain registrar and set your nameservers to some other DNS host, whether it be DDOS-Guard (which is what kiwifarms did with their .ru domain), the domain registrar's own, or any other. It will take a bit to propagate but should be pretty quick. Of course if you bought your domain on Cloudflare you may be kind of screwed, but I don't think that was so in this case.
EDIT: It was https://who.is/whois/kiwifarms.net so they might be screwed. Depends on if Cloudflare still lets them access the dashboard and transfer the domain to another registrar.