You cannot delete a user in Gazelle, but you can inactivate their user account so they can't log-in anymore.
FROM THE WEB APP
Step 1: Go to Settings > Users
Step 2: Change their status to "Inactive"
Step 3: Save
NOTE: This user will no longer be able to access your system. This applies system wide in both mobile app and the web app.
FROM THE MOBILE APP
This setting can only be changed by logging into the Web App at this time.