Siebel calendar access issues

We have a user (USER A) who had access to another user's (USER Siebel calendar.  USER B has left the company and all shared calendar permissions for him have been deleted.  USER A still sees USER B in his calendar owner dropdown, but every other employee that had access doesn't see him anymore.  I have tried granting and revoking the access again, but it doesn't make a difference.  How can I remove USER B from USER A's calendar droopdown?

Thanks in advance....