if you want this for the whole suit of armor and not just the chestplate would you have to do !=yourchestplate multiple times?
You would need to check every armor slot for allowing flight, and check all of them again until any one of them is incorrect to disable flight, at which point you can stop checking.
In logic / code terms, that's:
// for allowing flight
playerHelmet != null && playerHelment.getItem() == YourMod.yourHelmet &&
playerChest != null && playerChest.getItem() == YourMod.yourChest &&
playerLegs != null && playerLegs.getItem() == YourMod.yourLegs &&
playerBoots != null && playerBoots.getItem() == YourMod.yourBoots)
// success! every armor slot contains the correct armor piece!
// for turning flight off, you only need to find ONE incorrect piece, using either OR ||, or else/if statements
playerHelmet == null || playerHelmet.getItem() != YourMod.yourHelmet // notice how it's switched from the first example
|| playerChest == null || playerChest.getItem() != YourMod.yourChest
// if any single one of those statements is true, the player is no longer wearing the full set
// disable flight UNLESS the player is in Creative Mode