I'm trying to change the permissions of a specific forum so that the registered user group can only reply to topics and not create new ones. I edit the node permissions for the forum I want to change the permissions for and select the register group. Remove all permissions to post threads and moderate own content.
This then effects my administrator account as well as my demo account.
The admin after a new installation is actually a member of Registered. I am guessing that you set Deny permissions for Registered which would have overridden the permissions granted by the Administrative group. Try using Revoke instead of Deny.