I too had the same problem. I installed PC Suite at one time when I first got the phone and never used it since. My N900 (default) calendar was listed as "synchronized" vs "local" and it would not let me delete it. Also, in the settings -> Transfer & Sync showed a PC Suite connection configured but would not let me remove it.

I just started using Erminig to sync my calendar with Google Calendars although I had to create a new local calendar on the device for it to work.

I really wanted to remove the "Synchronized" on the N900 calendar and was prepared for any extreme.

I opened terminal and renamed /home/user/.calendar/calendardb to /home/user/.calendar/calendardb.old and rebooted.

When it restarted the calendar was empty, but showed local. I had to re-establish my birthday's "smart" calendar and then I re-synced using Erminig to get my calendar back from google. I still put it in a separate place as now I am very careful with how I treat the N900 calendar.

The other good news is the PC Sync account in settings -> Transfer & Sync was finally gone too.

This worked for me. Just posting for those who may be interested. If you break your phone don't blame me.