Circle .... I own a pony named zoom and he zooms when he gets scared so you should try to figure out the trigger and then figure out the sotution. For example zoom hates birds and when there's a bird around he will take off so I start to swig saw a with my hands and sit my a little but not so much that my horses is pushed forward
For a quick stop, use a one rein stop or circle! However, this should only be used in emergencies. Does your horse bolt on a regular basis? If she doesn't then a one rein stop or circle will do. If she does, then work on it! There are two ways I know how. 1.) Like mentioned earlier, mix it up! Do figure eights, circles, serpentines and lots of changing direction! She will never no what hit her. 2.) If she wants to run, let her run. Meaning, if she takes off, keep her going. When she tries to slow down, make her go faster. Keep her running until she is tired. Do this every time she takes off and she will learn that it requires more work for her to take off (because she won't get to slow down) than if she just behaves herself.
Hope this helps,
If you can't stop her use a pulley rein, but to keep her calm keep her guessing with different patterns. The reason for this is because if you keep her guessing and she actually has to think about what she's doing then it will take her mind off of bolting