Yes, eventually conspiracy theorists will likely believe the fact that foil hats do not protect the brain from radio waves. It's a popular scene in movies about aliens and space invaders: People wear foil hats to guard their brains from being tampered with. Movies love it, but science disproves its effectiveness.
It's true that people in noisy radio environments often wear foil hats to shunt radio waves away from their heads. While not even heavy brass foil will block a strong wave, almost any reflective foil can help reduce radio noise around the head, aiding clearer hearing and vision. However, conspiracy theorists are not wrong that radio waves can be focused and used to harass people.
Most people who believe these things will just think that this fact is just something made up by the government to get them to take off their tin foil hats. They will just start arguing with anyone who tells them something like the fact that tin foil hats will not protect them from radio waves.
Conspiracy theories will always believe that foil hats can protect from radio waves, because people like to believe what sounds good to them. People have thought lots of strange things over the years. For a long time people thought that the earth was flat. There are also other things that people believe now that are probably untrue.
Obviously a study that purports that foil hats do not protect against radio waves is only going to cause someone that believes the contrary to dig in further. People that reflexively believe something like foil hats protect them from radio waves are bound to find reinforcement from a study that find to the contrary. They will turn against that study and argue that the source is suspect and that the reason they are saying that is to try to get control of the Foil Hat Brigade.