Use the adjective resolute to describe a purposeful and determined person, someone who wants to do something very much, and won't let anything get in the way.
"Milo thought for just a moment and then, with a resolute 'I shall,' volunteered to go," Norton Juster writes in the classic children's novel The Phantom Tollbooth. From the way Milo said "I shall," we can infer that he was quite confident in his split-second decision, or at least made himself appear to be so. Some near-synonyms for this word include resolved, adamant, unswerving, and unwavering. If you make a "resolute refusal" of something, you're declaring you don't like it and never will.