Under the hood, on the left-side of the engine, on top if you look closely is a red marking on the linkage. Push and hold until it dies.
Or as someone described finding it "Open hood, stand with toes against driver's side front tire, and look down at the top of the engine. You should clearly see a red lever marked STOP about midway back atop the engine. Push the lever and the engine will stop."
You have a major vacuum problem. Sometimes its from the source, the vacuum pump, and/or bad lines.
CAUTION: Driving a car with disk brakes and no vacuum assist can be VERY scary not to mention danagerous. It requires unusual pedal pressure.
The hissing is telltale. Often it is simply easier to replace the bad line and/or fitting than trying to fix it. Obviously someone has already tried it and it has failed.
Here is one of many, many posts on the topic: Won't stop