what i did for WPM was to spray baking soda / water mixture onto the plant every few days on the leaves. do this in the dark period.
it wont kill it - it will come back. so you will have to keep spraying.
then you will have to Bud wash your buds with Lemon Water solution on harvest. you can google that one for specifics.
i had my whole tent (4 plants) all get WPM and this worked for me.
after that i got a better fan on my next batch and no more WPM