There is any way to disallow users from creating a sharing link and only permit share using emails?
I wouldn’t think so. Even with email sharing, a link is created and sent in the email.
A alternative is to use the app “guests”. Then you share with a guest (e-mail) and the guest must login with e-mail-address an self set password. But you can not deactivate normal sharing. Nextcloud is a file sharing plattform 
The user can download his data and share it with a free nextcloud if he/she do not like your nextcloud. Or he/she uses federated file sharing (e. g. free nextcloud https://cloud.tab.digital/ ).