Some advertisements are disabled when you subscribe to the Sonos Radio HD service, but not all, as some stations may still include Ad’s within their own streaming service, which helps pay to keep the Station/Staff in business.

One way to get rid of all Ad’s from a stream is, you may first need to find a streaming service URL that has no advertisements included and add that to Sonos using the method described in the below link…

Note some radio services charge for some Ad-free URL’s to be issued uniquely to their listeners and again that’s how the station may sometimes achieve their income. 

https://support.sonos.com/s/article/260