The Spotify Terms and Conditions of Use states the following:
"The following is not permitted for any reason whatsoever: artificially increasing play count or otherwise manipulating the Services by using a script or other automated process."
Therefore, using an ad blocker is prohibited.
Advertisements are what makes Spotify Free a possibility. Spotify uses ads to pay artists the money they deserve.
Have you tried Spotify Premium before? You might be eligible for the 30-day free trial.
Let me know if you have any other questions :)