pour carbonated water (seltzer water/club soda) into the tank. it wont kill the bacteria below the top of the substrate, but will kill anything that requires lots of oxygen(snails). it drops the PH so fast that the snails instantly start suffocating while their shells start dissolving. just turn off the filter and any source of flow, or they might be able to survive by hiding around the substrate. you will likely see them try to leave the water. if they do, just collect them up and dispose of them as you see fit.
when you want to add something to the tank later, you wont have to worry about any chemicals remaining that could harm your livestock.