I assume that you are using RAS. Check the documentation there's a call to disconnect a current connection. I last worked with this stuff a year or so ago and so don't remember the exact details, but it was very easy to do and was in the Windows documentation on RAS.
I do remember that it was all done through the command line because I had to come up with a way to trap the RAS responses and return them to LV.
I know this isn't a complete answer, but I hope it points you in the right direction.
Mike...