Click on the device (not the volume(s), which is/are listed below the device and is/are indented) in the left-hand sidebar of Disk Utility when you boot from the OS X Install CD, then click the "Partition" pane.
Select your partition scheme (1 partition, 2 partitions, etc.), and click "Apply."
This will destroy all data on the partitions and create a new, empty partition, requiring a reinstall of OS X. Be sure your data on the partitions is backed up.