I don’t have a complete solution. But, there are two comments.
For web browsers that let you add custom search engines, you can add a particular Mojeek Focus as a saved search engine to your web browser:
Then, using that custom search engine, you could permanently block up to twenty-five sites.
Second, the Mojeek API is more flexible than Focus. App developers could follow this example that Colin wrote in January. Though, this example is more about including websites:
Of course, you could also implement your own blocklist that filtered an unlimited number of sites from Mojeek’s API responses.