You can delete a volunteer ID from your Salesforce Account. 

From the V4S Mobile users tab, search for the volunteer name and click the dropdown button which as at the end of the row. in the V4S Mobile users list and click the delete button to delete it.

The deleted V4S Mobile User can no longer log in to the V4S Kiosk or Personal application on a Device nor Signup, Check-In or Check-Out of volunteer Jobs/Shifts.