First, try launching Outlook.exe with the “cleanreminders” command line argument:

C:\Program Files\Microsoft Office\Office12\OUTLOOK.EXE /cleanreminders

If that does not work, Then try the following:

Download MFCMAPI tool from http://support.microsoft.com/?ID=291794

-Run MFCMAPI on the desktop
-Go to Session->Logon and Display Store Table
-Select your profile
-Right click Mailbox and choose “Open Store”
-Expand “Root Container”
-Right click “Reminders” and select “Open Contents Table”
-New Window Launches with Title Reminders
-Select all the reminder items listed there, Right click and select “Delete Message”
-Close MFCMAPI