I would arrange for my target to receive a life time supply of Taco Bell... it's delicious flavor will draw him in... soon he will be eating nothing but toco bell. This will lead to weight gain, hardening of the arteries, diabetes, and eventually a devastating stroke that will leave him/her a shell of their former self. The targets family will have no choice but to pull the plug thus completing the mission with no fear of detection.
Now if you will excuse me... i have a sudden craving for a Chalupa and Quesodilla combo with a Baja Blast Mountain Dew to drink.