Next time they point out the empty spot on your wall calmly explain to them that you were thinking about mounting their grinning heads up there so you can be reminded of how happy they looked when you put them into those big fish that they have on their wall, which they never would have gotten into without your expert help.

That should pretty well shut them up, especially if you can pull off the straight face while they try and break the tension with laughter afterwards. People take you seriously when they think that you are nucking futs and say stuff like that. Trust me on that one.