[imp] How to disable spam folder?

[Leonardo]

Hi,

I tryed to find a configuration to disable autorcreation of spam folder and all spam fetures 
in IMP.

To disable all feature I puted the lines below in imp/config/pref.local.php:

$_prefs['*spam*select']['advanced']=false; 
$_prefs['*spam*_folder']['advanced']=false; 
$_prefs['delete_*spam*_after_report']['advanced']=true; 
$_prefs['empty_*spam*_menu']['advanced']=true; 
$_prefs['purge_*spam*_interval']['advanced']=true; 
$_prefs['purge_*spam*_keep']['advanced']=true; 
$prefGroups['*spam*report']=false;

Instead to try to prevent autocreation of spam folder I tried to use:

$_prefs['*spam*_folder']['value']=null; 
$_prefs['*spam*_folder']['locked']=true; 

but it doesn't work and I get an error on login tasks notifications.

The only option I found is to edit the file imp/lib/Mailbox/SessionCache.php removing 
IMP_Mailbox::SPECIAL_SPAM from the array in getSpecialMailboxes function but I would 
like to avoid  any code changes.

Are there any configuration alternatives? 

Thanks