What version of Moodle are you using? 1.8? 1.9?
If you have the Moodle logo on the bottom of your homepage, you can hover your mouse over the logo to reveal your version.
Here's some info from Moodle Tracker:
In Moodle 1.8 there is Grade exceptions tab under grade. The tab is easy to use and very intuitive.
However in 1.9 there is no tab for this function. The solution I found is that I have to manually input 0 in some student results who are excepted from some activities.
You can also exclude grades from aggregation in grade edit form - little edit icon next to each grade.