Since I don't know what you have tried, the first things to try are as follows:
1. Change the job's schedule: run on a given date, on hold, delete when complete, then try deleting the job.
2. From the Services window, stop the Backup Exec services, then restart them ... are you able to delete the job in question?
3. From the Task Manager, kill any Backup Exec-related services, such as beremote.exe, bengine.exe and beserver.exe. This may generate a prompt in Backup Exec asking if you want to close
4. Alternatively, you can try rebooting the server and see if you can delete the job afterwards (this is simply a different application of effectively stopping the service and killing the process).
It could also be a result of authentication issues; go to Network > Logon Accounts, find your Backup Exec service account and select edit, then change the password (change ... rekey the correct password). Then, from Tools > Backup Exec Services, select Services Credentials and rekey the Backup Exec service account information.