Blocking Broadband Access Between Certain Hours

How can you block a computer from accessing broadband between certain hours.
I have been told by Xtra New Zealand that there is a programme that can do this but they cannot tell me this because they do not sell the programme. Please advise of the programme name and where I can buy the programme from. The reason for this enquiry is because some carers I know want to block the access of one of their clients without disconnecting the Broadband Modem every night
Thanks

Robert