Does the 2004 cavalier have an anti-siphon valve between the filler tube and the gas tank?

No. There is an anti rollover valve in the gas filler neck of all modern vehicles that prevents a syphon hose from reaching the gas tank.

Most modern thieves will simply drill a small hole in the gas tank and take what they want (allowing the rest to drain out on the ground).

However, there is one trick that gas thieves can use to reach the gas through the fuel filler, but it involves using a small, still metal tube (think refrigerator icemaker water line tubing), cut at a sharp angle and twisted while it was being inserted. It takes a while to do this, and will take forever to syphon out just a gallon, but it can be done.

get a locking gas cap or install the anti siphon device that goes in the filler neck

Newer cars have valves in the filler tube which are rollover spill prevention and Evap valves which do not allow thicker tubes to enter into the gas tank. See these YouTube links as below which show you to how to Siphon Gas out of the Tank. Good Luck.
- https://www.youtube.com/watch?v=2mGeb9s6yFE

It's most likely the fact that there's an anti-syphon trap in the filler pipe.

Due to gas thefts becoming more prevalent when gas was either hard to come by or expensive or both, many manufacturers installed anti-siphon blocks in the gas tank filler necks of many vehicles. These can sometimes be gotten around by judicious fishing of the inlet tubing down through the gas filler neck, but it can be very hard on some vehicles. Try measuring the distance from the bottom of the gas tank to the top of the filler neck with a yardstick. You can hold the bottom of the yardstick approximately at the bottom of the tank as you can see it while kneeling down beside the back tire and note the distance to the filler tube. Mark off that distance from the end of the inlet tubing for the siphon and put a heavy mark on the tubing at that point. If you can\'t get the inlet tubing down the filler neck to this mark, you probably aren\'t going to be able to successfully siphon gas. You can try twisting the tubing as it goes down the filler neck and pushing it up and down to see if you can snake by the anti-siphon block. It helps if the siphon inlet tubing is small and stiff, but again, no guarantees.

Most cars have an anti siphoning device in the filler tube that resembles a screen so you can't put a tube down to be able to siphon gas out of the tank.

Anti siphoning device. Is fitted to most cars and vans now one way is to obtain. An electric fuel pump and disconnect fuel line at engine connect the electrical fuel pump externally to car anp pump out fuel into container

there are several things that can cause this. There is a vent hose or the main hose to the tank from the filler that could be kinked, collapsed orr plugged. There may be a emissions valve on the filler neck that is bad too, they are not easy to deal with because the tank should be emptied and the vehicle raised and remove the gas tank. Difficult to do and dangerous due to fire and explosion hazard.

I would try disconnecting the filler hose and see if you can bypass the anti siphon ball by siphoning out of the tank it's self. Most of the time there is a a rubber hose that goes from the filler to the tank its self. If that does not work you may need to remove the gas tank and drain it.

not 100% on this but i believe all cars after like 97 have an anti-siphon screen that keeps the tube from entering the gas tank.