Here are the steps for deleting rate packages for former students. If you've already created a rate package for 0.00 per month, you can skip steps 1 and 2, and just use the rate you've already created for the remaining steps.
Click on your name in the upper right corner, select Settings form the menu, then click on the Rates tab.
Create a rate that charges the student 0.00 for every 1 month.
Go to Students -> Students List and click Former so that only former students are shown. Adjust the number at the top of the list so that is says, "Show 100 entries."
Click the checkbox at the top to select all of the students listed.
Above the tabs, click the Update button to open the Bulk Edit dialog box.
Scroll past the list of selected students to the Status and select Former, to make sure these students don't get changed to Active.
Click on Rate Package and scroll down until you find the rate for 0.00 per month and click on that rate.
Click the green Update button to submit the changes.
Go back to Settings -> Rates and click the checkbox next to the former student rates you want to delete, then click the delete icon at the top of the list of existing rates.
If you select any rates that are still assigned to any students, the software will not delete them, but you will get a message telling you which students are still assigned those rates. You'll need to edit those students and assign a different rate to them before you can delete those rates.