To recycle isobutane canisters the leftover fuel must be emptied then the pressure released with a ‘crunch-it’ type tool. 
We weigh our canisters with an inexpensive kitchen scale before using and then after each trip to see how much fuel is left so usually we empty the canister cooking. Canisters with too little fuel for backpacking become car camping canisters. 
If a minute amount of a few grams is left, not worth cooking with, then that fuel must be burned off; just light the stove and in seconds the canister is safely emptied. The burned amount should be tiny, insignificant, but imperative for safely recycling.