1. Open Truecaller and grant the required permissions.
2. Tap Block (or the shield icon, depending on your version).
3. Turn on Block Top Spammers or Auto-block Spam Calls.
4. You can also manually block a number:
* Open the spam number in your call log.
* Tap Block.
5. To block hidden or unknown callers, go to Settings → Block → Block hidden numbers (if available on your device/version).