Can we restrict users from adding ShortPoint SPFx App to the site?

Yes, you can. 

To add the ShortPoint App to the site users need to have at least Read permission level. 

You can remove their permissions from App Catalog, if you do not want them adding ShortPoint App to the site:

1. Open the SharePoint Admin Center and then navigate to the SharePoint App Catalog (you may check Steps 1-2 of this solution article for more details).

2. Open Site settings. Choose Site permissions option under the Users and Permissions category.

3. Remove User Permissions:

• For SharePoint 2019 users' permissions are assigned manually by admin. So if you want to remove someone's rights, simply choose the user and click Remove User Permissions button from the Ribbon under the Permissions tab:

• For Office 365 there is an automatically created group Everyone except external users, that has Read permission level. It means that the users can add ShortPoint SPFx app from the Site contents by default.

  You need to remove this group in order to restrict them from adding the app:

For more details, check our solution: How to Restrict Users from Adding ShortPoint SPFx App to the Site?