So I've this code:

@ForgeSubscribe
public void preventInvalidBlocks(PlayerInteractEvent event) {
	try {
		if (event.action == event.action.RIGHT_CLICK_BLOCK && Block.blocksList[event.entityPlayer.inventory.getCurrentItem().itemID].getFlammability(null, 0, 0, 0, 0, null)>0) {
			int x = event.x;
			int y = event.y;
			int z = event.z;
			int face = event.face;

			switch(face){
			case 0:
				--y;
				break;
			case 1:
				++y;
				break;
			case 2:
				--z;
				break;
			case 3:
				++z;
				break;
			case 4:
				--x;
				break;
			case 5:
				++x;
				break;
			}
			if(event.entityPlayer.inventory.getCurrentItem().stackSize>0)
				event.entityPlayer.inventory.getCurrentItem().stackSize--;

			//It seems to be necessary otherwise the player wouldn't catch on fire when walking over it
			for (int j = 0; j < MinecraftServer.getServer().worldServers.length; j++) {
				MinecraftServer.getServer().worldServers[j].setBlock(x, y, z, Block.fire.blockID);
			}

			event.setCanceled(true);
			System.out.println("Invalid block placed!");
		}
	} catch (NullPointerException e) {
		System.out.println(e.getMessage());

	}
}

 

and it places fire instead the block. this part works well, the only problem seems to be reducing the size of the itemstack. I tried many ways and didn't succeed. What happens is that the stacks loses an item but it have it back immediately.

 

Example: I've 64 wooden planks. I place one on the ground and it's replaced by fire. The stacksize drops to 63 and goes back to 64.

Server side only.

You have to get EntityPlayerMP instance and use .inventory.setInventorySlotContents(slotNumber, itemStack).

You have to get EntityPlayerMP instance and use .inventory.setInventorySlotContents(slotNumber, itemStack).

 

How do I get the EntityPlayerMP from the event?

You have to get EntityPlayerMP instance and use .inventory.setInventorySlotContents(slotNumber, itemStack).

 

How do I get the EntityPlayerMP from the event?

 

I made it work like this

@ForgeSubscribe
public void preventInvalidBlocks(PlayerInteractEvent event) {
try {
	if (event.action == event.action.RIGHT_CLICK_BLOCK && 
		Block.blocksList[event.entityPlayer.inventory.getCurrentItem().itemID].getFlammability(null, 0, 0, 0, 0, null)>0) {
		int x = event.x;
		int y = event.y;
		int z = event.z;
		int face = event.face;

		switch(face){
		case 0:
			--y;
			break;
		case 1:
			++y;
			break;
		case 2:
			--z;
			break;
		case 3:
			++z;
			break;
		case 4:
			--x;
			break;
		case 5:
			++x;
			break;
		}

		if(event.entityLiving instanceof EntityPlayer)		{
			  
			  EntityPlayer player = (EntityPlayer) event.entityLiving;
			  //player.inventory.decrStackSize(player.inventory.currentItem, 1);
			  int newStackSize = player.inventory.mainInventory[player.inventory.currentItem].stackSize - 1;
			  
			  
			  for (int j = 0; j < MinecraftServer.getServer().worldServers.length; j++) {
				  if(player.inventory.mainInventory[player.inventory.currentItem].stackSize != newStackSize){
					  MinecraftServer.getServer().worldServers[j].getPlayerEntityByName(player.username).
					  inventory.decrStackSize(player.inventory.currentItem, 1);
					  
					  //It seems to be necessary otherwise the player wouldn't catch on fire when walking over it
					  MinecraftServer.getServer().worldServers[j].setBlock(x, y, z, Block.fire.blockID);
					  break;
				  }
			  }
		}
		event.setCanceled(true);
		System.out.println("Invalid block placed!");
	}
} catch (NullPointerException e) {
	System.out.println(e.getMessage());
}
}