Let the complex number \( z=x+iy \) be such that \( \dfrac{2z-3i}{2z+i} \) is purely imaginary. If \( x+y^{2}=0 \), then \( y^{4}+y^{2}-y \) is equal to:
\( \dfrac{3}{2} \)
\( \dfrac{3}{4} \)
\( \dfrac{2}{3} \)
\( \dfrac{4}{3} \)