Disallow subsite to be registered under main domain, possible?

Hi

I tried to search about it, and if I recall correctly I think this was possible in the past.

I have mainname.com and multi-domain with name2.com name3.com and I want to allow people to register subsite.name2.com or subsite.name3.com but NOT subsite.mainname.com

I tried to add the mainname.com to the multi domain settings with flag "private" (while the others are "public") but it still shows in the drop-down to create a site.

Is it possible with the plugin as is? or I must modify something, add a function somewhere, or whatever :slight_smile:

Thank you very much in advance for your help.
Best Regards
Patricia