Hello,
In the OCS console right click your Forest>Properties>Global Properties, then select the meeting tab. From there you can edit eht policy that is assisgned for meetings (Unless you've changed it it will be Default). Select the policy you would like to edit and hit the edit button. From there you will see the "Enable Program and Desktop Sharing" section, if you don't want control passed back and forth choose "Never allow control of shared programs or desktops". That should do the trick for you.
Thanks!
-KP
Kevin Peters
MCSE/MCSA/MCTS/CCNA/Security+
blog: www.ocsguy.com