have 5/8" T 1-11 on my shop built in 1989 - recently found a woodpecker hole and covered it with metal. Next week found another that I covered with 1/2" welded wire. I looked today and find another. What can I do? I'm thinking of trying a spray called Ropel.
Deterrents based on smell or taste (such as Ropel) won't be effective with birds, as most species have poorly developed sense of smell and they wouldn't engage with the taste aspect when they're pecking. Your use of metal flashing to block the started holes is a good idea, especially to prevent birds from nesting in the wall cavity. To protect a broader area, I would suggest using tightly-stretched lightweight bird netting to literally block access to the wall. Most installations would tightly tent from eaves to below the zone being exploited by the birds. Quite often people will wrap a lightweight pole of some sort in each end of the netting to make attachment of the "net tarp" easier. See Figure 3 in this source: http://icwdm.org/handbook/birds/bir_e139.pdf
I hope this information is helpful.