The above process by @lasd218 is no longer valid.
I Figured it out effective 5/30/2018.
1. In the upper right, by your User ID Tag, click the “bell” notification symbol.
2. Select the “gear” Icon. This will send you to the ‘Profile & Preferences’ menu
3. Select “Reminder”
4. Deselect email reminders for due tasks.
Also, I would go ahead and adjust your preferences for notifications as well. If you are using hubspot free, some of these notifications go against your maximum, and many of them are completely unneccessary. Make sure only the notifications you really want are selected to maximize your value.