Public Key Only authentication problem

I am trying to setup an FTP server using Serv-U that would ONLY require PUBLIC KEY authentication.

I've set the option in Limits & Settings | Password | SSH authentication type to "Public Key Only"
When I attempt the connection from the Unisys ClearPath MCP system client, the PublicKey authentication passes, but then there seems to be an attempt to do perform password authentication as well, which always fails.

[03] Thu 01Dec16 13:02:48 - (000022) IP-Name: <mydomain name> (<my ip address>)

[30] Thu 01Dec16 13:02:48 - (000022) SSH2_MSG_USERAUTH_REQUEST: user: xt11450837; service: ssh-connection; type: publickey

[02] Thu 01Dec16 13:02:48 - (000022) User "xt11450837" logged in

[30] Thu 01Dec16 13:02:48 - (000022) SSH2_MSG_USERAUTH_REQUEST: user: xt11450837; service: ssh-connection; type: publickey

[02] Thu 01Dec16 13:02:48 - (000022) Invalid login credentials; user: "xt11450837"; password: ""

[02] Thu 01Dec16 13:02:48 - (000022) Password wrong too many times for user "xt11450837" - disconnecting

[31] Thu 01Dec16 13:02:48 - (000022) SSH2_MSG_USERAUTH_FAILURE: login failed

[02] Thu 01Dec16 13:02:48 - (000022) Closed session

I've tried disabling the PASS command; I've set the user's password to an empty value and also to just a single character but even that fails.

Does anyone have an idea on how to get around this?