Access to Calendar

[RogerM]

I am a home user with a single computer using Outlook 2007. I have dealings with an organisation that is somehow able to add appointments to my calendar. I have not given permission for this and can't see how to prevent it. When I right click on Calendar in the navigation pane I do not get "Change Sharing Permissions", as the Help page suggests I should, but only "Open in New Window" and "Navigation Pane Options".

Can someone please explain step by step and in words of one syllable how I can control access to my calendar and prevent others from adding to it. Any help will be greatly appreciated.