Hi,
According to my understanding the soluiton is for overwirte the job rather than append the job is,
In Backup Job Properties, Media and Device under the Destination heading check the OVERWRITE MEDIA rather than "Append to media, overwrite if no appendable(which is selected by default)". that practice will overwirte the media rather than append.
For the Second problem the solution is ,
Right click the media set and move it to the "Scratch Media" after moving media to scratch media you will be able to delete the media sets..
I hope that will assist you. in the case of proble you are free to ask again.