OCS file sharing and desktop sharing blocking

you can do disable desktop sharing using a policy, create a policy and configure meeting poicy as per user basis, then when configuring the user select the ploicy that disables desktop sharing
http://technet.microsoft.com/en-us/library/dd425145(office.13).aspx
for file sharing I cannot remember if the meeting policy has file sharing option or not, will check it and come back to you, you can check it as well and let me know

actually it is file transfer option, i did find a policy to block file transfer but not sure how can you apply to selective members.

if the policy has file transfer option then create a new policy that has the required settings.
from the global settings select use per user policy
go to the required user and right click then configure OCS options.
then select the required policy

this policy is the template you can impose through group policy

no you don't use GPO to deploy it, it is a policy that is assgined to users in active directory users and computers or using OCS administrative tools