I like using the Hiya App. It is free and I haven't noticed any ads.
Not only can I choose to block certain calls. I can also report spammers, scammers, fake commercial charities and robocallers. Other users of the App also report those types of calls, so a lot of them are intercepted for me, and rejected by the App, without me ever having to take the call. You can also read the other peoples comments about the caller to find out what the scam call was about if you are curious about the call you missed.