Currently we don't have a field on sp_portal table to set active to false for service portal like we have for CMS.
However, we have some options to consider here.
1.One is to change the value of URL suffix to values other than 'sp' so that if the users tries to access 'sp', it is not a valid one. But, users will still be able to see the correct URL suffix if the user has the access to sp_portal table.
2.Other option here is to set the homepage for sp '404' page so that they see a page not found if users access the URLs (/sp) manually.