How to disable adding new terms via the autocomplete deluxe module for taxonomy terms?
You can use the content taxonomy module: https://www.drupal.org/project/content_taxonomy

On the content type fields that are taxonomy reference fields there will now be an extra option that says: Deny any new terms checkbox

It will throw this error when creating a new term: 

Client: new terms are not allowed. Please choose from the suggested options.



Apparently there are other solutions I found aswell:

https://www.drupal.org/project/taxonomy_autocomplete_permission -- but it isn't working for autocomplete deluxe.

And there were a few discussions where it involved hacking core to disable it. The content taxonomy module seems to be the only way. It would be better if we didn't have to install another module.