"No” is a complete sentence. Anything else either clarifies or amplifies your message. In terms of self defense, anyone who won’t take “no” for an answer likely doesn’t have your best interest in mind.
It’s okay to say No:
No, thank you.
No, I don’t want your help.
No, I won’t give you my phone number.
No, I can’t help you.
No, you can’t sit with me.
No, I’m not interested.
No, I’ll take the next elevator.
No, I’ll grab the next cab.
No, I don’t want to sit with you.
No, I won’t go up to your room.
No, you can’t join us.
No, I’m not okay with that.
No, you can’t come in.
No, I don’t want to share a cab with you.
No, you can’t use my phone.
No, you can’t call me.
No, I won’t accept your Facebook invitation.
No, you can’t join us.
No, I won’t loan you $20.
No, I won’t let you give me a ride.
No, I’m not going with you.
No.
Any of these sentences has the potential to save you from a compromising or dangerous situation, if you’ll simply say the words and mean them. And if the person doesn’t respond appropriately, know in advance what you’ll do next…and do it with conviction.