A bleach spray would work, but most roofers can install a zinc strip near the peak of the roof and as it rains, water will react with the zinc strip and kill the stain.
There is a relatively new product called"spray it and forget it" you just spray it on and leave it and it works over time. I haven't used it yet, so you should research their website first and get customer reviews. They sell it at Ace Hardware.
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