How do I prevent my calendar from synchronizing with yahoo?

Go to Menu > Tools > Accounts and click on your Yahoo! account. On the General tab, scroll down to Services and make sure only the IMAP and SMTP services are ticked.

Not sure which ones will be there, but in the above example, CalDAV syncs the calendar and CardDAV syncs the contacts. 

Thank you for your reply - I have tried that but this also prevents me from seeing the calendar on em client.  Is there a way around this?

Maybe I should rephrase the question; I want to use the calendar on my em client platform but do not want the calendar events to appear on Yahoo or my mobile devices - is this possible?

Yes. Remove the calendar protocol as I have described above. Now any events that you create will not be synced with other devices connected to the Yahoo! account.

If you want both the Yahoo! calendar and a non-synced calendar in eM Client, make sure that any new events are added to the Local Folders calendar, and not the Yahoo! one. You will see when you save an event, in the bottom left of the window, you can choose which calendar to save them in.

In this example the event will be saved to the Local Folders calendar, so it will not be synced. Clicking on the path you will be able to choose which to use.

Thanks again for your response which I think I understand.  However, having unticked the CalDAV box, the calendar has disappeared.  Could you please advise how I create a Calendar in Local Folders? (apologies, I’m a bit of a Luddite!) For clarity, I do not use the Yahoo calendar and only want a calendar on my PC.

OK, so leave the CalDAV unticked. That means no Yahoo! calendar in eM Client.

Now all you have is the Local Folders Calendar. If Local Folders are not visible, you can enable them in Menu > Tools > Settings > General > Show local folders.

Got it!  Thanks Gary, I appreciate the help.