N.P.
My kid does that every so often. But less now that she knows if she bugs me after I've said no, she'll lose whatever she's asking for for another day. If she bugs me about more TV after I said TV time is done. It's three strikes and you're out. The third time she asks I tell her not only is there no more TV today, but she doesn't get any tomorrow either because she won't stop asking after the answer was already given.
The next day when she asks to watch TV, I'd say, "No, because yesterday you repeatedly asked to continue watching TV after I had already said no and if you keep asking today, you'll lose out on TV tomorrow too."
Shuts her up pretty quick. It's important to follow through though. If you say NO TV TOMORROW because of the excessive badgering, then let her watch TV anyway, she'll know you're just full of empty threats and her behavior won't change.