I found an answer on the official website that requires no 3rd party software installation. You just have to change volume settings like described :
There is a workaround described above that stands:
Set the volume on the Spotify player software to maximum. Then adjust
the computer output volume back to a comfortable level. No more loud ads.
Problem lies in the fact that Spotify allows advertisers to adjust the
player volume, against your will, when their ad is played, overriding
the player volume setting you have set yourself. Naturally they set it
to max - because they can!
When you set the volume of the player to max yourself, the ads cannot
increase volume anymore higher than you now listen to your music at.
Effectively you have created a ceiling to the audio volume.
This results music and ads be played at the uniform level. And you still
can set the actual output volume to any level you like via the computer
Tested. I can confirm this is an effective workaround, that Spotify
cannot bypass without your consent.