No, he actually can't. You're confusing kbin, and kbin.social
He can absolutely choose to block people at the instance administration level, or defederate an instance, as the Owner and Administrator of kbin.social, or karab.in, or whatever other instances he might run.
There is no reasonable way, as the developer of the kbin software, that he can magically write into the source code that "nazis aren't allowed" for instance. How would you even test for that, when somebody downloads the software, and tosses it up on their own server? He can't control what people do with the software he writes, as long as they're otherwise complying with the license he releases it under.
Could he put "nazis, bigots, and other assholes aren't allowed to use this software" into the license? Probably, if he chose to, but it's basically unenforceable.