How can I remove virus from Levovo K3

I recently purchased a Lonovo K3 online, and it's been working fine up until a few days ago. When I checked my phone in the morning I saw that about 3 apps had downloaded during the night, and I was getting ads on the homescreen. It's gotten so bad that it's at the point where I can't even use the phone, there are just ads and popups everywhere, so I downloaded Sophos antivirus and ran a scan of the phone, and it said that the in-built security app was a threat, and that it was adware. Unfortunately I can't uninstall it because Android seems to prevent you from doing this unless you have rooted your phone, which seems impossible to do with ads redirecting you every 2 seconds. I have done two factory resets, with no luck. Whenever I turn the internet on, it will just started displaying these ads/popups and keep downloading unwanted applications.
Anyone have any idea what I should do to get rid of this?