Do you use a Gas Bandit Blocker to keep would-be thieves from stealing gas out of your car?

No. Don't waste your money. Thieves will just cut the supply pipe close to the tank to drain the fuel. This will cost you in time and repairs. If it's happening in your area keep a minimum amount of gas in your tank. That way they won't target you again for 1/2 gallon of fuel. Keep a full two gallon portable can indoors and take it with you on trips. Set up a video surveillance camera if possible too.

There seems to already be something to prevent putting a hose into the filler on my 2010 Corolla.

I put a DIESEL emblem on my car(one I got from a junk yard off another car that is DIESEL. I am not saying thieves can read but for those that can, they scope out the car they are going to steal gas from during the day light hours.