У меня этот код работает 

Код
type
TRasConn = record
Size: DWORD;
Handle: THandle;
Name: array[0..20] of AnsiChar;
end;

TRasEnumConnections = function(var RasConn: TRasConn; var Size: DWORD;
var Connections: DWORD): DWORD stdcall;
TRasHangUp = function(Handle: THandle): DWORD stdcall;

function DisconnectDialUp: Boolean;
var
Lib: HINST;
RasEnumConnections: TRasEnumConnections;
RasHangUp: TRasHangUp;
RasConn: TRasConn;
Code, Size, Connections: DWORD;
begin
Result := True;
try
Lib := LoadLibrary('rasapi32.dll');
try
if Lib = 0 then
Abort;
RasEnumConnections := GetProcAddress(Lib, 'RasEnumConnectionsA');
if not Assigned(@RasEnumConnections) then
Abort;
RasHangUp := GetProcAddress(Lib, 'RasHangUpA');
if not Assigned(@RasHangUp) then
Abort;
FillChar(RasConn, SizeOf(RasConn), 0);
RasConn.Size := SizeOf(RasConn);
Code := RasEnumConnections(RasConn, Size, Connections);
if (Connections <> 1) or (Code <> 0) then
Abort;
if RasHangUp(RasConn.Handle) <> 0 then
Abort;
Sleep(3000);
finally
FreeLibrary(Lib);
end;
except
on E: EAbort do
Result := False;
else
raise;
end;
end;



Пример использования: Код

if DisconnectDialUp = true then
ShowMessage('Соединение разорвано')
else
ShowMessage('Не удалось разорвать соединение');