Answer
The IFS runtime client stores the login screen's remembered username and related settings in Windows IsolatedStorage, under a folder like <user profile>\AppData\Local\IsolatedStorage. Inside that folder, look for a set of generated .txt-named files — one of them holds the cached username and a few other login-related settings. These files aren't meant to be directly edited, but deleting the relevant file(s) clears the cached username, which is useful when a shared ClickOnce launch scenario like this incorrectly carries over or mixes up which user's cached login details get used. There's no standard supported way to grey out or suppress the Extended Logon Options specifically, but since that section is normally hidden by default unless a user has actively used it before, clearing IsolatedStorage this way effectively resets that hidden/shown state back to default as well.