I'm not assuming they're hitting Ctrl+C.

What I'm saying is the screen that says

                               VIM - Vi IMproved                                
                                                                                
                                version 8.0.567                                 
                            by Bram Moolenaar et al.                            
                  Vim is open source and freely distributable                   
                                                                                
                            Sponsor Vim development!                            
                 type  :help sponsor<Enter>    for information                  
                                                                                
                 type  :q<Enter>               to exit                          
                 type  :help<Enter>  or  <F1>  for on-line help                 
                 type  :help macvim<Enter>     for MacVim help

disappears as soon as you enter insert mode (or really interact with Vim at all). If you're in insert mode, you don't see this. So if you see this, typing :quit will work.