Meaning of "scour"
Solution 1:
This is not an accepted use of the word "scour". The author doesn't know how to use the word properly. He is probably using the word in what he thinks is an archaic sense in order to make his book sound more medieval, but he gets it wrong. I suspect he may be mixing up "scour" and "scourge".
Solution 2:
One of the uses of the verb scour comes from a tradition of cleaning pots and pans, using a metallic brush or cloth and scrubbing vigorously, effectively inflicting intense punishment. I suspect the use of the verb in your book comes from that idea.