First, let me say CONGRATULATIONS on your fantastic success so far! Regarding your question, yes, there is a way to change the way exercise is featured into your goal. Changing your account settings will make it so that your nutrition calorie range doesn't change each time you exercise.
Instead, the nutrition tracker looks at your fitness goal for the week (how many total minutes or calories burned you plan to exercise that week) and calculates that into your range for each day. Because of that, it is important with the other method to make sure your fitness goals are as close as possible to the activity you actually complete each week to ensure you are eating enough to sustain your activity.
To switch your calculation method, you can go to your account/email settings from your start page and scroll to the section titled "How My Calorie Range Is Determined" and follow the prompts there.
To read more about the pros/cons of both methods of calorie calculation, this blog post has an in-depth analysis of the methods: www.sparkpeople.com/blog/blog.asp?post=new
I hope this helps!