How to set repeating times?

I've got a bookings calendar which is linked to multiple colleagues calendars to allow bookings to be made in blocks of 90 minutes (10am, 11:30, 13:00, 14:30, 16:00). These 5 times are the slots we would like to show.

However, with some staff, if they have other meetings/ appointments in their calendar, it shows the only other available 90 minutes they have free (e.g. 13:55).

Is there a way to only allow the times we want, and if they are busy at those times display them as not available instead of a random time during the day?

Thanks