First, identify the "horn relay" ... usually shown in the fuse/relay layout in the owner's glove box manual ...and be prepared to remove during next event.
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If that stops the horn... you must next determine if the relay was "stuck" or was energized by a voltage to the coil. If the relay is not at fault ... the next likely fault source would be the horn switch in the center of the steering wheel. Be careful in this area ... you don't want to have the air bag blow!!