you could use something like a kill switch of the type used on quads and the like attached by a lanyard to the ingnition key.
They are designed to be pulled out and kill the engine if the rider falls off so something like that wired into the fuel pump supply etcas previously suggested.
That way if the ignition key is removed and the killswitch key frogotten it will just pull out and if you use and elasticated lanyard it will twang back and give you a smack on the knuckles to remind you to remove it next time
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