Finally i've found a workaround. Maybe it's not as elegant as it would be, but for me it works. It resets the undo flag of an edit control.
Make a method with following code and call it at the end of TextChange eventhandler:
#if TargetWin32
Declare Function SendMessageA Lib "User32" Alias "SendMessageA" (hwnd As Integer, wMsg As Integer, wParam As Integer, lParam As Integer) As Integer
Declare Function SendMessageW Lib "User32" Alias "SendMessageW" (hwnd As Integer, wMsg As Integer, wParam As Integer, lParam As Integer) As Integer
// EM_EMPTYUNDOBUFFER = &hCD
If System.IsFunctionAvailable( "SendMessageW", "User32" ) Then
Call SendMessageW(Self.Handle, &hCD, 0, 0)
Else
Call SendMessageW(Self.Handle, &hCD, 0, 0)
End If
#endif