I'm not sure what method you used in the past but here are some solutions.
The easiest would probably be to use an app specially for stuff like this. A popular one is Mr. Number.
Not sure if your phone has this, but on some Androids, you can do in in the Phone app: Tap the more options icon () next to the microphone. Then, select Settings > Call Blocking and add the number you want to block. You can also go to Call history or Recent calls and tap the number you want to block, then select Block. Here, you can also Report call as spam.
On other phones you can from the call log, you can disable incoming calls from specific numbers. Select the number you want to block, then hit More or the 3-dot menu icon in the upper right corner and choose Add to reject list. This will disable incoming calls from specific numbers.