Staff turnover is pretty common in the tours and activity industry. Make sure that you set up a separate administrative access for each of your staff members. This way, when they leave at the end of the season (or for whatever reason) you have the ability to remove their access from the system.
To delete a user from your account, follow the steps found here.
The user will no longer have the ability to log into your Rezgo account.
Note: If you only have one user for your account, change the password on your account and make sure that the email address for the account is YOUR email address and not that of the employee that you wish to restrict.