Как программно отсоединиться от Интернета.
Добавьте на форму элемент CommandButton.
При нажатии на кнопку происходит вызов функции HangUp, которая закрывает соединение с Интернетом.

Const RAS_MAXENTRYNAME As Integer = 256
Const RAS_MAXDEVICETYPE As Integer = 16
Const RAS_MAXDEVICENAME As Integer = 128
Const RAS_RASCONNSIZE As Integer = 412
Const ERROR_SUCCESS = 0&
Private Type RasEntryName
dwSize As Long
szEntryName(RAS_MAXENTRYNAME) As Byte
End Type
Private Type RasConn
dwSize As Long
hRasConn As Long
szEntryName(RAS_MAXENTRYNAME) As Byte
szDeviceType(RAS_MAXDEVICETYPE) As Byte
szDeviceName(RAS_MAXDEVICENAME) As Byte
End Type
Private Declare Function RasEnumConnections Lib "rasapi32.dll" Alias "RasEnumConnectionsA" (lpRasConn As Any, lpcb As Long, lpcConnections As Long) As Long
Private Declare Function RasHangUp Lib "rasapi32.dll" Alias "RasHangUpA" (ByVal hRasConn As Long) As Long
Private gstrISPName As String
Public ReturnCode As Long
Public Sub HangUp()
Dim i As Long
Dim lpRasConn(255) As RasConn
Dim lpcb As Long
Dim lpcConnections As Long
Dim hRasConn As Long
lpRasConn(0).dwSize = RAS_RASCONNSIZE
lpcb = RAS_MAXENTRYNAME * lpRasConn(0).dwSize
lpcConnections = 0
ReturnCode = RasEnumConnections(lpRasConn(0), lpcb, lpcConnections)
If ReturnCode = ERROR_SUCCESS Then
For i = 0 To lpcConnections - 1
If Trim(ByteToString(lpRasConn(i).szEntryName)) = Trim(gstrISPName) Then
hRasConn = lpRasConn(i).hRasConn
ReturnCode = RasHangUp(ByVal hRasConn)
End If
Next i
End If
End Sub

Public Function ByteToString(bytString() As Byte) As String
Dim i As Integer
ByteToString = ""
i = 0
While bytString(i) = 0&
ByteToString = ByteToString & Chr(bytString(i))
i = i + 1
Wend
End Function

Private Sub Command1_Click()
Call HangUp
End Sub