Question: I'd like a list of the files (not lines) in a group of txt files that don't contain a certain character. Is this possible?

Answer: 
I'm currently not on a Windows machine so it is somewhat hard to test. However you should be able to use the [*find in files*](http://docs.notepad-plus-plus.org/index.php/Searching_And_Replacing#Find_in_files_specific_features) feature in Notepad++ combined with a regular expression using something like `\A[^@]*\z` where `\A` matches the start of the file and `\z` the ending. `[^@]*` says that everything in-between can't be an `@` char. Alternatively the regular expression `(?-m)^[^@]*$` should also work. Where `(?-m)` toggles off the [multi-line mode](http://docs.notepad-plus-plus.org/index.php/Regular_Expressions#Search_modifiers). With the multi-line mode off `^` and `$` should match the start and end of the files respectively.
Use the correct tool for the job, in this case the tool is `grep`. See the documentation on line. If you are on windows you could get [Grep for Windows](http://gnuwin32.sourceforge.net/packages/grep.htm), [Cygwin](https://www.cygwin.com/) or if on Windows 10 you can get hold of `Debian Linux` from the app store.