We believe our 3 year old son learned to be scared of monsters from a really cute library book we got several months ago (The Monster that Ate the Darkness). He'd been watching Monsters.inc for well over a year, and we don't talk about monsters at home.
We tried to convince him that we monster-proofed the house when we had it built, that both our cat and dog would eat a monster.
Finally, about 2 months ago, we told him that Monsters aren't real. If you ask him who the real monsters are, he'll tell you Cookie Monster, Mommy Monster, Daddy Monster and him.
We just have to keep reminding him that they're not real.
I don't know if I'd personally try the Monster Spray for concern it would reaffirm that they may actually exist when you want the opposite.
Good luck. That's a tough one because their imaginations are so vivid at this age.