Can you say?

How about " Not a chance"?

You've got to be kidding me! No way, no how! Are you crazy? Not for a million dollars! You really think I'm that stupid? Or, if you want to be polite: I don't think that's something I can fit into my schedule. That's not something I see myself doing. No, but I hope you find someone who can help you.

it depends upon what it is you have to say no and what would be its consequence doing so. For example suppose my colleagues say come to the corporate party and I am not willing to go :I say you people reach I will join you at the venue, and I do not join them until next day. Here consequence of saying flatly no is that you have to explain them why I am not coming with them.

You can say it indirectly, "sorry, no", "thanks but no" etc.