Well you just want them greyed out, right? You don't want them hidden.
The first thing you need to do is copy the contents of event_calendar/templates/Layout/Calendar.ss and CalendarEvent.ss to your mysite (or theme dir) /templates/Layout. Silverstripe always looks in your theme directory first for a template and falls back on the module default if it's not there.
Run a flush, and then in your Events control you could just do something like this: