By adding an offset in the settings for the Contact node, you can force the two sides of the solid to experience contact at some distance before they meet in the geometrical sense. This approach only will, however, leave a thin channel through which the fluid can pass. The reduction in flow may be sufficient, but you can block it even further by increasing the viscosity in the channel when the gap is closed. To do that, you can, for example, compute the minimum gap anywhere in the contact pair, and then make the viscosity a function of it. Another option is to compute the wall distance in the fluid from both sides of the contact pair and use that information to modify the viscosity. Do not increase the viscosity more than a couple of orders of magnitude, to avoid numerical problems.