Before you start
Things you need to know:
After performing this action, you have 30 days to recover (undelete) your contacts from the recycle bin, after which time their data is permanently removed. However, undeleting contacts won't put them back in the lists they were in previously.
You can only delete all contacts if you are an account owner or if you are a user with the Can manage account permission.
Delete all contacts from your account
To delete selected lists and all contained contacts from your account:
Go to Audience > Lists. Do not select any lists.
Expand the MORE ACTIONS drop-down menu, then select Delete all contacts from your account.
Read the information in the Delete all contacts popover, and, if you agree, enter the word delete into the text field and select DELETE ALL CONTACTS.
All of your contacts are deleted from every list in your account, including the Contacts master list and the system Test list.