We don't want users accepting any external invites to external company shares but we want them to be able to share folders with any company. Basically, we only ever want to be the folder owner.

We have a rule that only we can create the folders and share them out but this sometimes is accidentally overlooked. In addition, Box does not have an option to "block outside collaborator invitations" only an option to completely turn off inbound/outbound sharing or white list domains.