Why are blocked magazines still showing up in Random Threads?

I saw a thread from a certain magazine in my Random Threads and I decided to block it, but the thread remained even after refreshing multiple times. And when new random threads were shown, they pulled some from magazines I already blocked (I checked, I blocked those instances and it wasn't just a magazine from a different instance with the same name). Why is it doing this? This seems like it goes against the whole purpose of blocking an instance.