Yes, you can. Go to the class page, choose 'Actions', then 'Class grade settings'.
On the Class grade settings page, you can see the total number of activities for each material used in the class, and which ones are contributing to the grade. (All activities contribute until you choose to edit this.)
Click the ‘Edit included activities’ or ‘Edit included tests’ button to change the items contributing to the score.
Uncheck the activities you wish to exclude and save your changes. If you wish, you can preview activities by clicking on their titles.