As far as i'm aware OpenVPN uses users that are created on the system, So if you login to OpenVPN with the login of 'Darryn' there will be a system account named 'Darryn' so you will just have to delete him with the command 'sudo userdel Darryn' and this should prevent him from using the VPN software again. (You can test this just to be safe by creating a new user with 'sudo useradd Billy' and set a password with 'sudo passwd billy'.
And to list the users just use the command 'cat /etc/passwd' will generate a list of all accounts including default system accounts such as ftp. To output the list with just the names use this command 'cat /etc/passwd | cut -d ":" -f1'
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