Are there ways to fully dissociate a complex salt?
Let's consider the formation constant of this ion complex at $25^\circ\mathrm{C}$:
$$\ce{Fe^{2+} + 6CN- &lt;=&gt; [Fe(CN)6]^4-}\qquad K_\mathrm{f}=10^{+24}$$
This means that, the equilibrium is highly displaced towards the formation of the  complex ion. The ion complex is very stable.

So, if we want to displace this equilibrium towards the dissociation of the complex ion, we have to add an adequate quantities of  a metallic ion or a ligand that form with $\ce{CN^{-}}$ or $\ce{Fe^{2+}}$, respectively more stable complexes.