You can partially accomplish this by using the Tag keyword and the Search Exclude keywords, like so:
Search Exclude [Field A], [Field B];
Tag Fields [Field A], [Field B] With '$hidden';
The Search Exclude keywords allow you to prevent specified fields from being discovered in the Smart Search feature. The Tag keyword allows you to add the special '$hidden' tag to specified fields to prevent them from being found in the Sheet view. Note that while the hidden fields won't show up in the Assets pane or in the expression editor, they can still be used if the user knows the field names and has Edit rights to the sheet or the ability to create their own sheet in the published app.
If you want to be able to utilize fields in the app while still preventing users from having any access to them, you would need to create all necessary metrics in the load script and then drop those fields. Otherwise, if some users need to see those fields and others not, you can utilize Section Access to set that access.
