A free program for Android, by NetQin.

Never be bothered by unwanted or annoying calls with Call Blocker – Blacklist. With this app, you can create a blacklist containing all the numbers of people you don’t want to talk to at the moment. The whitelist, on the other hand, lets you include contact details from people you often want to hear from. 

Further, this tool is handy for detecting spam calls, reducing your chances of being scammed and giving you much-needed peace of mind. You can also block messages or show notifications after a call has been blocked under the “Settings” menu. Call Blocker – Blacklist can be downloaded on most Android devices so long as the OS version is Android 4.4 and newer.