To begin, hover your mouse/pointer over your Name in the upper left-hand side of your Mastermind.com dashboard.
From here, click the "Chat Notifications" option:
Once you are on your Chat Notifications page, find the little blue slider that says "On", and click it:
When you click it, the button will then become red and it will say "Off". This is confirmation that your chat message notifications have successfully been turned off.