A device for arresting spark and ash issuing from, and precisely controlling the ventilation of and temperature inside, a heat source-containing vessel such as a cooking grill, stove, oven, cooker, fireplace, heater, or firebox. The device comprises a spark arrestor assembly including a base plate, a cover plate, and a spark arrestor. The base plate defines an opening. The cover plate and spark arrestor are slidably mated in two channels formed in the base plate around the opening. The cover plate and spark arrestor may be adjusted to extend across or only partway across the opening as selected by a user of the device. Tabs are provided on the cover plate and spark arrestor for easy adjustment thereof. Tabs in the base plate are positioned to define a range of slidable motion of the cover plate and spark arrestor. The spark arrestor assembly engages a fire-containing or heat source-containing vessel over an opening defined by the vessel so that the vessel opening cooperates with the spark arrestor assembly opening.