iCloud pop up won't go away?! Help!

I bought an iPhone 5 off craigslist not too long ago (about a year) and recently it's been asking me for an iCloud passowrd to an account I've never even heard of. I'm guessing it was the iCloud of the previous owner. Up until now it's been working fine but now that I've been experiencing these issues I can't even use the phone because I have barely a second in between each pop up. I've tried contacting the person I bought it from but it seems that they have changed their number and email and all other sources of communication. I don't know what to do, I really need my phone to work. My friend was able to fix it for me last weekend, but it died on my again this morning and it's happening again. It happens every time I restart the phone. I already tried the thing where you click reset but don't actually reset anything and then click home button. This worked for me for some time, but now instead of the little window that pops up it takes me to an iForgot website and when I clcik home, my efforts are futile. The pop ups still appear. I'm about in tears now because I'm so frustrated. Someone please help me. Thanks in advance for anyone who is able to solve my issue. I am truly grateful.