Phil
In your situation I think the best you could do is not using media sets so the backup is done on the mounted tape (unless your customer has an autoloader). To do so, go to Tools -> Options -> Media Management, and set the Media overwrite protection level to None so that Backup Exec does not overwrite protect any tape. This could be dangerous: if your customer forgets to change the tape any day, that tape will be overwritten with that day's data.
Another option is using media sets. To do so, set the Media overwrite protection period to partial, and medify the defaukt media set with an Append period of 0 (none) and an Overwrite protection period of 6 days, it's what best suits in your situation.
Regards