7 day policy

I bought a cellphone, which I charged the whole night without using and put the phone on the next morning, without using the phone and the battery went flat the same afternoon. I went back to exchange the phone and the lady that assisted me told they cannot take it back because the battery of the phone has no guarantee and when I said I do not want the phone anymore she told they cannot take it back. WHAT AM I SUPPOSED TO DO WITH THE PHONE THAT IS NOT WORKING FOR ME.