Put your finger on the entry in the Recents and drag to the left all the way. You should see 3 buttons; orange, blue and red. The orange button adds the number to the blocked list.
Perhaps you have a good reason, but there is no point in blocking spam calls, because the phone numbers are not real, and the spammer will never use the same number again to call you.
Call blocking is for avoiding someone you know, or a real business you don’t want to hear from; not for blocking numbers that will never call you again.