No.The key signature prevails and must have it key rules respected ie the various types of scales, like Major, Melodic Minor, Harmonic Minor and numerous other exotic etc scales on could get adventurous composing music in eg Phrygian etc modes, pentatonic scales etc, like some rock musicians, Flamenco, Middle Eastern, indian etc music sometimes experiments with, to create unique sounds (that will give joy to some listeners, annoy others...).your accidental tells you to shift the note one or two half tones up or down but only for that bar, or until you hit the "natural" accidental sign, telling you stop playing alter notes of the basic key signatures time pattern, just play what the key signature says to play. So, if the key signature involved an F#, as the Popular D major guitar key does and an accidental told you to play this F# as a flatted F#, you'd simply play a plain F note for that bar. But as soon as the "natural" sign cancels that, in the next bar, usually, then you just go back to obeying the instructions of the Key signature for D major, which requires all F notes to be played as F#. Takes a while to get one's head around all this music theory stuff. But worth knowing it.