When you want to turn off the Johnson outboard motor on your boat, you have several different ways of "doing the deed." Two of them should almost always work, one should work every time and one works if you have no other choice. Your motor should shut off at demand; if it doesn't, something is wrong, whether it's a problem with a ground wire, a switch or the power pack that controls the operation of the motor.
Shift the motor into "Neutral."
Turn the ignition key switch to the "Off" position.
Pull the safety lanyard that's attached to the metal tab that connects to your throttle control, which pulls the tab from the "kill switch" on the control.
If the motor continues to run, press the plunger for the choke solenoid or push the manual choke lever to choke the motor out.
You can replace the ignition key switch yourself but if the ignition switch or the kill switch doesn't stop the motor, the problem is almost certainly in the power pack. Take the motor to a dealer for diagnosis and service before using it again.
Don't continue to operate a motor that can't be shut down. Too many things, like a propeller strike injury to swimmers, can happen when you go boating with a motor that's not in safe operating condition.