Tell them if they are caught using hangouts they will be written up and can lose their job 😉
Seriously though, blocking applications like that can be very difficult because they use so many different domains and IP blocks to operate. And whatever they use now could change without notice.
If you really want to try to do this at the Meraki level your best bet is probably to run packet captures while using Hangouts and see what common URL's and IP addresses you may be able to restrict without breaking other services.
Good luck.
- Ex community all-star (⌐⊙_⊙)