In an upcoming update to Google Drive, Docs, Sheets and Slides, Systems Admins will be able to set expiration date for specific user access to files in one of the above-mentioned apps. The scenario is quite simple. If a project lasts a month, you will be able to grant access to contractor for the duration of the project. After completion of the project, access to the file will be automatically denied to contractor if he/she tries to access it again. However, according to Google, "you’ll only be able to set expiration dates for users with comment or view access; you will not be able to set expiration dates for file owners or users with edit access."
Note that this "expiration date" feature is intended for business only and will not be available until mid- to late Q2 2016.