How do I disable the add button to add new items?

I don’t want my employees to be able to add parts, I only want it done via the spreasheet on the back end (it has to be done in several places). How do I get rid of that + option to add in the lower right corner?

You can mark the table as Read Only in AppSheet. However, it is not advised to edit the spreadsheet once you start using it in AppSheet. Especially if its multiple people editing that spreadsheet.