Here is the logon screen before the registry key has been added:

Here is what happens if a user selects the "Log on using dial-up connection" checkbox before logging on:

Setting the following registry key will disable the checkbox, disabling all users from using the dial-up connection feature:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

you will need to manually create a new string value (REG_SZ) called RasDisable. Set the value's data to 1.

Here is the final result:

And finally here is what I wrote to add the regkey to all computers using a KiXtart startup script through Group Policy:

;===============================================
;===============================================
;
; DISABLE LOGON USING DIAL-UP CONNECTION
;
;===============================================
;===============================================

IF READVALUE("HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon", "RasDisable") <>"1"
WRITEVALUE("HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon", "RasDisable", "1", "REG_SZ")
ENDIF

NOTE: I've noticed that the quote marks are not displayed properly on this page. Here is the KiXtart script in its intended form:

Download Here