My pick would be one of the following script driven solutions:
1. Create two scripts. One that moves the contents of the ibay to another location and the second to move it all back again. Create cron jobs to run the appropriate script at either end of your required times.
2. If you IBay is accessed via a CGI interface (PHP, Perl etc.) you may want to add a quick generic check to the start of each CGI file. If it detects it's outside the required time, it forwards users to a 'No access' web page.
Regards,
Luke