imac keeps asking me to sign-in to old Apple ID?

imac (2 years old) is repeatedly prompting me by pop-up to enter the password to a no longer used apple id, saying that my session with iTunes has expired.

When I enter my current apple id and password into the pop-up, it tells me the information is incorrect. I have also tried to enter the old password and old apple id into this box and it tells me that this information is also incorrect...

This is driving me crazy! It is prompting me over and over, please help!