How to use new boot drive and ignore/erase old one

Hi,

so I purchased a new SSD a couple of weeks ago for my boot drive, and with my luck it was defective. So I requested a replacement and I have cloned the old boot drive onto the new one (the replacement).

Because I have to send the defective SSD the back, I am trying to erase the content on it, however disk utility wont let me do it thinking it is still the main boot drive.

So basically, I currently have two boot drives, and I want to erase the partition on the defective SSD. I have set the new SSD as the startup disk but that didn't help.

Is there another way to tell OS X that the new SSD is the boot drive that matters so I can delete the old one?

Thanks for your help and sorry if what I described doesn't make sense lol.