Google Drive files -- Remove someone's access / prevent ex-assistant from accessing docs/sheets
First you need to know the email address associated with their Google Drive / Docs account. Then go to your google drive and use the advanced search (click the search options icon) and find the "shared to" field and put in their google drive email address. Then select the files from which you want to remove their access (to select all, CTRL-A on Windows, CMD-A on Mac) and then click the Share icon, and change them from Editor/Commenter/Viewer to "Remove".