My answer is not specific to your exact model, but with most mowers, your problem will be something to do with the kill switch mechanism. Check the cable that runs from the handle bar to the engine is not kinked, rusted internally or getting caught somewhere. When pulled, it will acvitavate a lever on the engine and when released a spring on this lever will hold it in the engine kill position. Take a good look, lubricate and check spring is present and working. You could disconnect the cable at the lever end and activate by hand to see if this stops the engine.
Rupert
December 2019