Prevent users sharing documents within a subsite?

Hi,

 

I have created a sub site and I have invited/given access to some organisational members (synced from on premise AD to Azure AD) and some external users to the site.

 

I want to ensure that no staff members share any of these documents with any other external users and I want to retain control of this.

 

Can I create a configuration which still allows external sharing, (I need to share some folders with external users), but doesn't allow organisational members or the invited external users from sharing with others that I wont know about? i.e. Only site owners can share with "Specific people"? see attachment..

 

How can I have this set up?

 

Thank you for your time,

Ollie