They might reply with some form of "But why?" in which case the best answer is - "this is none of your business" or - "because I want to". If they're extreme

They might reply with some form of "But why?" in which case the best answer is - "this is none of your business" or - "because I want to". If they're extremely manipulative, they'll follow up with more "why?"s but you just keep repeating these two until their why's are exhausted or you leave with a "So... bye" This technique is called the broken record in the book "When I say no I feel guilty" which talks basically only about these types of interactions.