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.