You can go to each of those challenge pages (Challenges tab - SparkChallenges - See my current SparkChallenges - then click on the specific challenge - click on the gear icon on the right side of the page and uncheck the box for Send me a daily motivational challenge update and click the button to update your mail settings. You will need to do that for each challenge you want to stop the emails on.
"Obstacles don't have to stop you. If you run into a wall, don't turn around and give up. Figure out how to climb it, go through it, or work around it." - Michael Jordan
Searching for a buddy? Try using the search feature in the SparkPages section.