The web applications generated by Appizy are client-side, therefore it works on the visitor’s browser. You cannot hide anything from your visitor.
In terms of security it is slightly more complex to steal than if you would propose a direct file download of your original spreadsheet. But to make a comparison it is like music or a video on Youtube. As long as you have the right skills, or the right software it takes 2 minutes to download a “protected” content.
You can reduce the access to your web-application by adding a layer of authentication on your website to only user you trust.
You might want to make the code unreadable in case someone tries to understand it. Several obfuscation tools are freely available. However, it is important to note that it is impossible to obfuscate the code to the point where it cannot be reverse-engineered.
The more advanced solution to hide all formula and data is a backend application. In this is case, the visitor only loads the interface of the application. The formula runs on a distant server. With a PLUS plan Appizy can generate such NodeJS application out of the box. But it comes with extra complexity and setup costs. You might need the help of a NodeJs developer.