Apple wont fix Iphone because 'been tampered with'

Im really struggling with what or where to go from here... And it seems a lot of people are having the same problem... My iphone 5c is still under warranty and after taking it to the Apple shop 3 hours of waiting the man comes back and says sorry I cant replace the phone as its been tampered with... I was given no proof or written evidence to tell me why/how or anything... What am I to do now? Is this apple just trying to pass my warranty on? It seems apple are very good at selling there products but not taking responsibility afterwards? Anyone had this issue and what was done? Ive been told I need to have it collected and taken in for testing... Im unsure whether this is just a waste of time to have my phone returned and be told again its been tampered with.... Well yes by your staff!!