You can block a phone number in multiple ways. The easiest is maybe to search for the contact and select 'block' in the detail view.
If you want to block a number in your call log, tap and hold on the number and tap on the block symbol, or you can go to Truecaller in-app settings> Block>Tap on the floating plus sign and can select 'Block a number' > it will ask you to enter the number details and you can add the details and can press on the block option that will make sure that the number is blocked.
Block from the detail view
