Then pen him up so he will no longer be able to harrass your dog.
If he continues your dog might (rightfully) snap.
And I would get him at least three adult hens for company.
Maybe relocate the rooster to freezer camp as it will take many months until the chicks are ready for rooster attentions. Among your current chicks there probably will be a male or two anyway.
And welcome to the BYC community! :frow