Do you have it pulled up in your own instance?
I found his profile by putting https://mastodon.satoshishop.de/@mk into the "Search or paste URL" bar in the top left of the web interface on my own instance, and was able to block the domain once I had his profile up.
(This should really be easier; they should add a way to just manually type in a domain to block on your "Blocked Domains" page.)