This is actually true

But you can just say "thank you :) you are beautiful yourself" not "what are you talking about, I'm so ugly"

well some people seriously don't believe it

Then don't say anything.

There is a big difference between being stuck up, being modest, and just flat out fishing for complements. "Ugh, I know Im hawt, don't talk to me" is stuck up. A facebook update of "Ugh look at this picture of me, im so ugly!" is flat out fishing for compliments. This is completely different from somebody saying "Oh your so pretty" and you disagreeing with how pretty you are