Hello Lakshmi,
It has been a while since you have made your post, in case you still need support I will be glad to assist you further. I suppose the other community members can benefit from this answer as well. I have been looking into your post and I created a sample project for you with the functionality you want. Basically I handled the DataManager’s ActivityAdding event and check whether the Activity’s Start Hour equals 12PM and cancel the event if it is.
Feel free to write me if you have further questions.