Both Lemmy and Mbin lack the ability to filter out or block Cloudflare nodes. They both only give a way to block specific forums.
Lemmy lets you block whole instances, it was introduced in 0.19.0 (which was released just before Christmas, but many instances didn’t update until 0.19.3 was released around the start of the year due to federation issues with 0.19.0).
I don’t get why you want users to be able to apply cloudflare filters, though. If your instance doesn’t use cloudflare, then you won’t access through cloudflare. I’d actually be really interested in understanding why this is something you’re looking for, rather than just the ability to block an instance such as Lemmy.world.