Coming to an end of a project or relationship or some activity, one could politely or sarcastically say: "It's been a pleasure". In this situation, saying: "It hasn't." negates the politeness.

"It's been a pleasure" is a common sentence used to describe an experience that has brought one pleasure. ** We often say it after meeting someone, either for the first time or not, and talking with someone for awhile and enjoying it. Or even working with someone and enjoying it. So, "It hasn't" means "it has not been a pleasure." Or "It has not caused me pleasure."

"It hasn't" means "'It has not been a pleasure." Hope this helps, John