The engine is not ruined. Diesel is more related to oil than gasoline. You'll need to drain the tank, fuel lines and replace the fuel filter. Since your car is still running, you may not need too much more done. Still, it isn't a cheap fix, maybe about $400.
The reason your car smokes is because you're burning a mix of gasoline and diesel (like a two stroke engine).
Don't let anyone tell you that the engine is ruined. Your car still runs, so your fuel pump is still working. Hold off on that replacement until it can be proven that the fuel pump is bad. Don't let anyone tell you that the fuel lines have to be replaced. Fuel injectors are apparently working since your car is running. Faulty mechanics will try to bleed you dry of your money through scare tactics.
If it were THAT important to keep diesel out of gasoline engines, the would have set up separate diesel pumps away from the gasoline pumps (like they do for compressed gas). Although gas engines are not designed to run on diesel, it's not a major malfunction.
