You can poison fish food. To get fish food and poison: get the
Poison in the room next to the kitchen in Draynor Mansion. The fish
food is also at draynor mansion floor 2.
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To get the poison you need to go to the small room immediately
south of the kitchen in the Ground Floor. It's a green bottle on
the floor.
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Or tell him not to came again, or else tell him to go in the
kitchen and see what happens.