Calendar rules

Is it possible to add feature to make a calendar date and time unique, so you cannot use the same date and time if it already has an event? Or to give an error message that the particular date and time is already scheduled?

I’m planning to use the Calendar View to allow users to Create New Events. It looks great.
However, I would like to limit the dates on which users can create new events - as the events can only be created on dates that have availability.
I would also like to set the background colour of the dates (eg) Green on dates when the user can create a New Event and Red on dates when they can’t.