How do I block a zombie tornado?

Unluckily you can't. You should plant wisely to avoid great damage. I also recommend you to save plant food to feed peashoters to kill the zombies that reach your plants.

For those, the best defense is a good offense. Fortunately the zombies that swoop in aren't that powerful. By the time they come in, I generally have two offensive plants on each lane (I like the kernel-pult with repeater, usually). That's usually enough to take care of them while the rest of the wave is still behind the wall-nuts.

Just accept that some parts of the game aren't about total defense, but adapting when things go wrong (see also: zombie parrot).

Nobody said it in an answer yet (spartacus did discuss it in a comment):

Plant your attack plants (shooters) in the back and your sunflowers (that you hopefully need less of by end of game) to the right of them. Then the zombienado will only eat "less useful" and cheaper sunflowers.

The best solution is to put plants that don't shoot or throw anything. Bonk Choy and Snapdragons are the ones Ive used. I notice the Jester will walk right up and not twist or spin.

Squash will take out the zombie tornadoes.

Well, To defeat it, You have to use Area of effect plants or Instant kills in order to beat it.

Here are Plants that will get deflected by the Jester Zombie:

Peashooter

Cabbage-Pult

Kernel-Pult

Melon-Pult

Winter Melon

Coconut Cannon (It cannot deflect the Plant food effect on it)

Citron (It cannot deflect the Plant food effect on it)

Pea Pod (It cannot deflect the Plant food effect on it)

Bowling Bulb (It cannot deflect the Plant food effect on it)

Pepper-Pult (It cannot deflect the Plant food effect on it)

Fire Peashooter (It cannot deflect the Plant food effect on it)

Red Stinger (It cannot deflect the plant food effect on it)

and others.

Now here are plants that can kill the Jester Zombie:

Bonk Choy

Snapdragon

Spikeweed

Spikerock

Lightning Reed

Iceberg Lettuce (Prevents the zombie from deflecting the plants)

Chili Bean (Prevents the zombie from deflecting the plants)

Stunion (Prevents the zombie from deflecting the plants)

Laser Bean

Magnifying Glass

Fume Shroom

Guacodile

Banana Launcher and Endurian.

Hope this helps :)

The mature form of the bomb plant can kill the tornado zombie.

You dont have to worry about that you just need to be prepared with chard guard in the area were the zombies will arrive then the zombie will be pushed in front of the wall nut but make sure your wall nut is close

What I do is I use the pumpkin and imitator version to protect the plants that the tornado blows the zombies to. Or you can have slowing plants that make the zombies in the tornado slower and the tornado too. If all fails, then try to pile enough firepower on them and they will die.