How can I block certain numbers from calling my Call Routing number?

To block certain numbers from calling your call routing number just go to "Call Routing" and expand the dedicated configured number.
Then go to the bottom and enable "Blocked Numbers"

Here you can add or more number(s) that are not allowed to call your number.

You can also work with wildcards. An example would be +49 344* which blocks +49 3441, +49 3442, +49 3441 1234 and so on.