What is the best way to douse a charcoal grill?
Add some coals over the hot ones and/or reduce airflow some other way (many grills have slits or vents to control airflow, play with those, close them partially).

Both should reduce heat output (adding coals of course will mean the grill will burn for longer and will eventually heat up again).

I'd not throw water or other liquids on it. Only makes a mess :)