Hey there, Summer.
To turn off the notification of PMs, you go to "My Messages" and once you are in that part of the boards, you click on the link at the top marked "Preferences". Then select "Change Settings". From there, you should see a drop-down box that is next to the option "Notify by email every time you receive a personal message". Just set it to "Never" and you should be good to go.
I hope that helps!