if you want to reduce the evaporation of VOCs from the NAPL phase, you may consider to reduce their vapor pressure which is computed either by the Wagner or Antoine eqs (eqs. 4.3.10 and 4.3.11, respectively, in the TMVOC user's guide of 2002). You may chose the eq. coefficients in such a way that resulting Pvap is negligible.
But you should also consider that in G+Aq, G+N and 3-phase conditions the concentrations of VOCs in the gas phase are primary variables. If you need to simulate such conditions, and in particular 3-phase conditions as I'm suspecting, I'm not sure what will be the impact of negligible VOC concentrations in the gas phase on the convergence process. I guess Pvap of a few Pa will give you negligible concentrations in the gas phase at atmospheric P, but could still allow to describe the phase composition and solve the thermodynamic equilibria.
You should try and observe the code performances.