I think I just found out the solution.
1. create a .php file in folder: /custom/Extension/modules/Accounts/Ext/Layoutdefs/, like: _overrideAccountSubpanel.php
2. copy /modules/Accounts/metadata/subpaneldefs.php's contents to your php file.
3. remove this line(Actually, I don't know whether this step is necessary or not):
4. add a new code at begin(this code will reset all system subpanel setup)
if(!defined('sugarEntry') || !sugarEntry) die('Not A Valid Entry Point');
5. then, you are free to edit below layout_defs to you want.
$layout_defs['Accounts'] = array();
6. don't forget 'Quick repair'.
btw, you can find all about 'custom folder' here: http://developers.sugarcrm.com/docs/...4.html#9002252