You can start with MS Graph Files API which allows you to list the files and delete them. The files APIs give you access to the one drive as per the permissions granted by the signed in user.
Note that on Graph Explorer you can only do this for the current signed in user. To do this for all users in our tenant which is what I understand you mean by all staff, you need to use application permissions. Checkout Quickstart: Get a token and call the Microsoft Graph API by using a console app's identity.