Some phones have a built-in call reject feature. The option is found by tapping the phone icon to bring up the dialpad. Tap the menu icon (three dots in upper right of screen or tap More), select Settings and look for call blocking. The option may also appear in the Phone settings menu.
Just a quick warning if you just a call blocker app; most will simply direct the blocked call to VM, which is fine, but a few will automatically answer the call and immediately hang up. This works great when you're in Canada, but if you're traveling it can lead to high roaming charges. Simply something to be aware of.