If you would like to prevent anyone on you site from following members on your site, you can hide the "Follow User" links that appear on Member Profile pages and in the Member Snapshot by adding the following Custom CSS to your Display Settings.
#watchedUserSidebarDiv, #followUserUserDivAction { display:none; }
Copy and paste that code into the Custom CSS box in your Display Settings control panel, which can be found here:
Manage > Admin Control Panel > Settings > Display Settings
Once you add that custom CSS, the Follow Member links will no longer appear on your site. Note that this also means that if someone is currently following a member, they will have no way to un-follow them, either.