Conservative economists simply assume recessions don't exist, that they can't exist, that there are no idle resources, no involuntary unemployment. Since everything is already optimal and in equilibrium, the economy is a zero sum game and any attempt at change can only make things worse. Because of this no stimulus is even possible as it can only redirect resources from their current use to the use of government. There is even a bit of truth in this during normal times when the economy actually is operating at capacity. There is no cure for recessions because they are figments of our imagination. It certainly saves a lot of work trying to predict, explain, and remedy them. In this bizarro world, there is no output gap, empty homes and idle factories are just speculations for higher future prices and the unemployed are just enjoying leisure. Putting more people to work is just denying them their leisure and recessions are just long awaited and much desired vacations. If you believe this, not only is fiscal stimulus impossible, even monetary policy is unnecessary so one has to ask them why it should be pursued. One can only imagine what other innovations they might suggest, an end to unemployment and welfare, balanced budgets, and the rest of liquidationist policies.
Now over the long haul stimulus would not be stimulative because the economy would be operating near capacity and it would just redirect otherwise occupied resources within the economy, possibly to less desirable ends. But the same is not true in the short term if there are idle resources. In fact, if Ricardian equivalence holds there is little reason to expect people have not already taken these government stimulus actions into account in their planning from the start and have planned for the government to do just this for them in a situation like this. Call this the Hyper-Ricardian Hypothesis. In that case to not do it would be disappointing expectations that had been previously established. Far from people countering fiscal action to make it ineffective, they will have planned for it and be relying on it to smooth their consumption. Lack of stimulus could mean they may not be able to.