User registration on subsite in multisite configuration

Question

As a network admin in multisite configuration, I have the option to enable or disable the user registration \ site registration. is there any way i can enable the user registration on s single sub site , not on all sites.

In my WP setup, I have a main blog which is managed by me and I dont want user to register here. i have a another blog installed on sub directory dedicated for forum. I would like to enable user registration here, so that user can use the forum