The two previous comments are not really the opposite of "Thank you" ... since, in common with the preferred reply, they express an honest and sincere (albeit arrogant and self-indulgent) response.
Far more hurtful is complete indifference ... that's what I think is the opposite.
During the TV show, The Dragons Den, the Dragons told a woman her idea was rubbish and her presentation was also rubbish. She looked close to tears.
She replied with "thank you"
Obviously "thank you" was not what she wanted to say to the Dragons, "stuff your program and your money up your @rse" would, I think, be nearer the mark, but what IS the opposite of "thank you"?
"No thank you" does not seem to fit the bill.
'Thank you for your valued critique and input. Please may I suggest you crawl under this stiletto so I can squash you like the bug you are and wipe you off before I step indoors'
bednobs is closest. The Sex Pistols did an interview with Bill Grundy in 1970s which offers a choice of answers, too.
Bob Geldof famously interrupted Andy Kershaw during BBC Live Aid broadcast to say to viewers "Give me your f****ing money," which is as close to thanks as Sir Bob usually gets.