Lemmy and kbin are two different forum software that can be installed and run on servers. Because both use the ActivityPub protocol, the content between them can be shared. So, a Lemmy user will be able to see content from a server running kbin, using Lemmy.
There isn't an option, you can't even tell but you already have the kbin content.
You can't disable it.
The reason that's happening right now is because kbin is enacting ddos protection using cloudflare so they aren't federating properly, this is a temporary problem.
Is this only some kbin instances thing or all kbin instances thing? I'm fairly sure that fedia.io (/kbin instance) does not use cloudflare. But that would definitely explain why I'm struggling to search some instances that might use it.