When you re enroll families, the grade is automatically updated to the next grade for a student.  If there is no available grade, the student's grade will be N/A.  This is usually a graduating student.  These are the steps to remove those students.

In the Pending families view:
  1.  Click Enrollment > Pending Families
  2.  Click Grade
  3.  Select grade N/A
  4.  If needed, scroll to the bottom of the page, change Show Rows to Select 100
  5.  Change View Mode from family view to student view
  6.  Click the check box next to the word Family to select all students in this view
  7.  Click Select Action
  8.  Click Delete Students
  9.  Click Submit

How do I manually rollover a family account from one school year to the next?
Refer to Knowledgebase article How do I manually re-enroll families?