How can I block an entire instance on kbin?

Next to each thread's post, you should see an instance specified in parentheses, such as (lemmygrad.ml). Click that item to list other threads from that host, as well as a sidebar box that says Domain. Click the "no smoking" type of button to suppress everything that domain produces. I think.

Verify it worked by going to your user's settings menu, click the blocked tab, then the Domains link.