I have a couple of sites where Backup Exec does not overwrite the files on the backup media. They use removable USB devices, either Dell RD1000 or WD Passports. I constantly have to manually deleted old back ups.
In Device and Media section of the job, I have it set to "Overwrite Media." the media set is configured to overwrite protection period 1 hour
I change the settings in Tools / Media management - Media overwrite protection level to "None" and unchecked "Prompt before overwriting"