In newer ispCP versions you can disable strong passwords (the enforcement of characters AND numbers) in the GUI: log in as admin and go to Settings → General Settings.
To disable them in older versions you have to do this: open /var/www/ispcp/gui/include/ispcp-lib.php and change the following option:
// enable or disable strong passwords // false = disable, true = enable $cfg['PASSWD_STRONG'] = false;
You can also set the minimum password lenght.
// password chars $cfg['PASSWD_CHARS'] = 6;