Is it possible to configure VIM so that when an attempt is made to edit a file encrypted by VIM, or to encrypt a file, VIM would warn or refuse such action?
I think you should build your custom version of Vim excluding this feature (
FEAT_CRYPT) completely. This is the most complete and error-prone solution.
To make Vim fail on attempt to write encrypted data you can try something along those lines
augroup example | au! autocmd BufWritePre * if !empty(&key) | throw "No encryption allowed here!" | endif augroup end