How can I stop Calendar from importing work events?

I use iCalendar as my main calendar, but I also subscribe to my work-based Microsoft exchange account on iCalendar. I also use the Mac OS Outlook email app.

iCal has started automatically importing some (but not all) work events as 'read only'. I cannot delete them. For some, even if I have RSVPd no, I cannot remove them from my calendar. I do not know if the source of these is subscribing to my exchange calendar to iCalendar, using MS Exchange for mac as my email programme, or both.