The WinSCP application have the updates disabled by default actually.
It's the WinSCP installer that enables the updates by setting the
, if you keep the default "Enable automatic updates ..."
installation task. So just turn the task off and the updates will be disabled too. This obviously won't affect the existing users anymore.
You cannot have WinSCP hide the Check for Updates
This request has been added to the tracker:
Bug 1413 – Administrative restriction to disable check for updates
You can block the update check URL on your firewall though.