Thanks to all who responded. I appreciate all your help.
We intend to use moodle for online training exams for the time being. Trainers would be having a live discussion with the trainees before taking the exams. I would suppose for now, we just want to hide or prevent any new editing trainers from initiating forums. We however, might consider using forums in the future if deem required.
Since the 'forum' cannot be hidden from the list of activities after all our attempts, we may just likely to orient new trainers not to start forums. The student nevertheless has no access to see messages so this has added security as well.
Thanks again for your support. Happy moodleing to all!