Good question.
Unfortunately, I don't have the answer for the web client.
I assume that you don't want to totally suppress (hide) books with only the format.
In the PC version: in Calibre, you would simply remove the tick next to the format in
Preferences: Behavior: Use internal Viewer for... (that passes the book to the OS for handling)
The Web interface is still evolving, so maybe Kovid will be willing to make it mimic the main Calibre