no, unless they're an a**hole
No should be sufficient. If someone presses you then you should say, "I'm sorry but I am unable to accept your invitation. I just want to make sure that you heard me say no, Thank you." As long as you are firm in delivering those two sentences stressing No Thank you then it should be clear that you no longer care to discuss it.
Yes. Even my old supervisor would :P
They drill you because they want you to accept.