Phone call Spam list?

I add all unwanted callers to a contact called "Block." In the settings for that contact, select "All calls to voicemail."
 
Mr Number is a better solution because it has crowd sourced SPAM identification as well as your own block list.
 
I add all unwanted callers to a contact called "Block." In the settings for that contact, select "All calls to voicemail."

Most telemarketers won't leave voicemail so that works well. You can also find a ringtone specifically for that contact that's simply an mp3 with a few seconds of silence just in case something glitches and you get one unwanted ring before voicemail picks it up.

Also I'm pretty sure there are a couple of apps in Play that will actually totally block the number period, but all the good ones need to be running as a background service to work.