But this persons posts are still showing up? Shouldn’t they be blocked?

The way I understood the Blocked Instances setting to behave, I would have thought so. But as Blaze responded in that Meta thread, what this setting does is suppress that instance’s communities from your feeds - not its users (though there is a separate setting to block individual users that I have employed and can confirm it works as advertised). That seems a bit half-hearted of a feature. Hopefully Lemmy will at least offer you the option of blocking the users as well as the communities in the next release (due “soon”^tm^).

In the interim, the advice in the linked post suggests using an app instead of the main Web UI, I use Boost personally and can see it has a block instance feature. I haven’t played with it, though.