remove 'sites' from buddypress

How do I remove the ability for members to create 'sites' once they register. I only want to use buddypress as a forum in 2 groups.. each of those groups should be able to keep their discussions private from the other. (new persons register, ask to join the appropriate group, and seek support and ask questions from the members of that group).

http://www.oppencas.ca

Hope I've explaining this correctly. This is the first time I've used buddypress so any tips would be greatly appreciated.

Thank you,
Malinda