When you try to close the throttle you may find that it will not turn or the engine will not slow down. Here is what to do:

Relax and let up on the throttle.
At the same time, pull in the clutch and turn off the engine with the kill switch.
If the motorcycle does not have a kill switch, pull in the clutch and let the engine race until you can stop, and turn it off with the key. You may also be able to leave the clutch out and stop the engine with the brakes.
Park the motorcycle until you can get it fixed.​

http://www.nbcnews.com/id/35783011/ns/business-autos/t/how-stop-your-car-when-throttle-stuck/