downing a masqueraded link

Hi all. I need to drop a dial-on-demand pppd link between 6pm and 8pm every night for security reasons (eg deny users from accesing the net between those times). I cant find a good way of doing this in smpppd, only using diald, which i dont want to use. What i'd like to do is set up a cron job to disable masquerading at 6pm, and bring it back up at 8am, thereby disabling the isdn link at those times. How would i do this? Or is there a better way of doing this? Thanks all, Elf.