As far as I know, SSI is enabled on all accounts and can't be changed with out manual editing of configuration files.
You can set whether an account has access to php by editing his plan and checking the box next to enable php. These checkboxes are also provided when creating plans, and customizing user plans during their creation.
If you are the customer, you would need to contact your hosting provider. Otherwise both reseller and admin levels have the capability to grant or revoce php privileges.
View list of all users -> View profile of user in question -> Modify plan for that user with php checkbox checked.