In Backup exec console in the Media tab
You can create different media set where you can defined the overwrite protection period and append period .
Once you configure the Media set you need to select the media set in the backup Job properties so the tape is protected as per the media set properties .Refer the following document this will gve you better understanding about media mangement in Backup exec
http://support.veritas.com/docs/192265
If you want your Backup job to Run on specific tape you need to create partiton in the Library
and when configuring the Job you need to target the job to the partion in which you have the tape on which you want to run the backup Job .
refer the following document as well
http://support.veritas.com/docs/299171