Sure!
I’m talking about this post by @Filmos
Where hidden formulas are listed (thanks also to @Paul_Danyliuk) i don’t remember now where i read that those could be used to turn the doc in a unusable app, for example after a verify from a license checker, but they could technically be used, apart from the fact that they could stop working like expected in anytime and break docs.
This is not a viable method because of the unstability of those formulas
Coda should work on protect user doc letting other users use those.
Like i was saying, a published doc with a save button (with copy blocked and play mode) would allow us to share our work with just listed people, that are not allowed to edit the doc, but that can use certain fields or control to interact with it!
But you already have figured out this! 
I’m preparing a new post with my proposal of payment management in coda, to integrate also license management, to help shape this feature for future 
Then, once i know that user is not one of the paying one coda should give us the tools to block that doc
I also have 2 other suggestions, i should be able to view, under the publishing statistics, WHAT are the (in my case) 6 copies of my doc, where they are, who own them ecc, and also i want to know if my ip is counted on the views of the doc (Probably yes because i’ve got 160 in 3 days ahahaahaha )
Nice to see others dealing with public app on coda 