Can you check the advanced editor section for the datetimes and make sure the each datetime is connected to only one ticket? This way going forward new registrations will have the correct datetime access.
With regards to bulk cancelling registraitons, this is possible via the Event Espresso > Registrations page (the screen options panel located on the upper right corner of the window will give you the option to load more than 10 registrations per page).
It’s also possible to use the Batch email feature to send out bulk notifications to selected registrations. More info here: