WORKAROUND (these steps may have to be performed regularly as they do not stop the issue re-occurring):

Warning: Incorrect use of the Windows registry editor may prevent the operating system from functioning properly. Great care should be taken when making changes to a Windows registry. Registry modifications should only be carried-out by persons experienced in the use of the registry editor application. It is recommended that a complete backup of the registry and workstation be made prior to making any registry changes.

1. On the Backup Exec server, manually create the HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\SymVmTools\SymVmToolsBackupTaskQueueTimeMax" key as a REG_SZ value.

2. Set the key value to a date/time string value representing the 24-hour date/time ("M/D/Y HR:MIN:SEC") of the "Requested Start Time" that is seen on the "Recent Tasks" list on the VI Client of the "Symantec backup" job to be canceled. Only US English 24-hour date/time format is allowed, such as "10/9/2011 17:45:11". For example, if the "Requested Start Time" of the "Symantec backup"
job to be canceled is "11/15/2011 1:22:50 PM", then enter "11/15/2011 13:22:50".

3. On the Backup Exec server, manually create the HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\SymVmTools\SymVmToolsBackupTaskQueueTimeoutWindow" key as a REG_DWORD.

4. Set the key value to a recommended setting of 4, which will cause "Symantec backup" jobs within 4 hours of the time set in step 2 to be canceled. 

At the next browse, Backup Exec will scan the vCenter database for "Symantec backup" tasks that were created by the current user, that are incomplete, and have a start date/time that falls within the time window that was specified by the two keys.

If "Symantec backup" tasks that meet these criteria are found, then the tasks will be canceled. The key values that were set in steps 2 and 4 will be reset. If another task or block of tasks need to be canceled, go to step 2 and skip step 3.

In vCenter, the tasks are marked as canceled by user and are cleared from the display list.

Note: If the value of the key is entered incorrectly in the steps above, only the display of the job is removed from vCenter. There is no effect on the Backup Exec job itself.