Yeah, should have been more specific: I restrict a Joomla article category via access level. That category contains articles extended by seblod cck. So subscribers are in a different user group than non subscribers.
" the 403 error is throws by Joomla core and we don't have control over it"
??? Of course you do. That is what plugins are for. Your plugin needs to check for the ACL. That is the main purpose of MSpro - redirect users with no subscription to the plan page when they try to access restricted content.