Are you trying the Reverb Depth or the Reverb Type? Sorry, I'm not sure how they would be listed in the settings so it might not say the word "depth," but the Reverb Depth controls the amount of the Reverb effect which is applied to the voice, while the Reverb Type selects between different varieties of the Reverb effect.
You might also try adjusting the Chorus Depth and Chorus Type.
Finally, there might be a DSP effect, or possibly even DSP 1 and DSP 2 effects, which can be set to any of the available Reverb or Chorus effects in addition to many other types of effects, so you might also need to adjust the DSP Type(s) and DSP Depth(s).
If nothing seems to help, it might be that the sound of the actual instrument was sampled with a certain amount of reverb "built in," such as being sampled inside of a room which was designed to give a rich, natural reverb to the instrument. If that's the case, it might be impossible to get rid of the "built-in" reverb.