Newbie with privacy issues

I have two questions that I'm thinking are probably simple solutions but I just don't know how to do it.

1. I'm using the guestbook module and the private message module. Is there any way for users to block other users from posting in their guestbook or send them private messages?

2. I've implemented Organic Groups but the edit tab shows up for users who have just added the group. They're able to modify the name, description of the group, or even delete the group just like the manager who created the group. How can I fix it so that tab does not show for regular group members?

Any help is greatly appreciated.