ange-ftp-default-user is a variable defined in `ange-ftp.el'.
Its value is

User name to use when none is specified in a file name.

If non-nil but not a string, you are prompted for the name.
If nil, the value of `ange-ftp-netrc-default-user' is used.
If that is nil too, then your login name is used.

Once a connection to a given host has been initiated, the user name
and password information for that host are cached and re-used by
ange-ftp. Use M-x ange-ftp-set-user to change the cached values,
since setting `ange-ftp-default-user' directly does not affect
the cached information.

You can customize this variable.