Determining Enthalpy Change of the Combustion of Ethanol
I see your concern, and it's absolutely valid. Any time you perform a calorimetry experiment, you need to be certain that you're not introducing extra energy into the system. Typically, the spark used to ignite the sample has a negligible energy input, but it is still there. However, any heat released by the flame (other than the initial energy put in to generate the spark) is generated by the combustion process. Without the combustion, the flame would cease to exist. If you were running this experiment, you could use an electric sparker and measure the power input for the sparker to know how much energy was inputted. Another option would be to run a control experiment with just an ignition (but no ignitable material) and see how much, if any, the water heats up. 

Of course, if you used a lighter to start the flame, and then held the lighter open throughout the experiment, you would be introducing extra energy from the combustion of lighter fluid, and this would give you false results as you feared. That's why we don't run calorimetry experiments in that way.