Since 1.7 added the ability to bind sprint to a specific key, I cannot seem to find out how to stop players from sprinting. This is how I am doing it:

 

public void handleLivingEntityUpdate(LivingEvent.LivingUpdateEvent event) {

        if (event.entityLiving instanceof EntityPlayer) {
             event.entityLiving.setSprinting(false);
        }
}

 

But this only appears to work if the player uses double-tap forward to sprint, not if they use the key bind to sprint.

What you said made me think of something I should have checked earlier. It looks like it is because Minecraft/Forge determines if the player is sprinting AFTER the Forge hook for LivingUpdate. So regardless of what you set player.setSprinting too, it will be redetermined once the Minecraft code runs to determine it.

 

This worked as override:

KeyBinding.setKeyBindState(Minecraft.getMinecraft().gameSettings.keyBindSprint.getKeyCode(), false);