Though I don't have that particular phone, I have some experience with the Android OS on multiple platforms.
Try looking up the target number in your call log from your calls menu. Press and hold the target number, and a list of options should appear with one of them being to block or auto-reject the call.
Another option is to follow this sequence of taps: Menu>Settings>Calls>General>Auto Reject
The Auto Reject list might provide an option to manually add the target number into the list.
Keep in mind that sometimes the version of Android running on the device might relocate some of those menu options, so try to use discernment in your navigation of the menus.
Here's a possible third option: your service provider might be able to block the number on their end - which would be the most permanent choice. Even if you change to a completely different phone, the target number would still be blocked. Try discussing the issue with a service representative of your provider.
Let me know how it goes.