This thread was a lot of help. Blocking extension to extension did not work for me though. I created a new context in extensions_custom.conf called [From-Internal-new] and added that context to a test extension. I then copied everything from the context [From-internal-additional] located in extensions_additional.conf. I started commenting out lines to see which one would stop extension to extension calling. The 2 lines that stop this is ext-findmefollow and ext-local.
I will play with this more in the future, but it worked for stopping extension to extension calls. You will probably have to make a custom From-internal like context for each group and a custom ext-local like context for each of those groups.