If my charcoal bbq is too hot and the chicken is turning stiff on the surface, can I close the top vents to cool down the barbecue?
What is the best way of cooling I down? Once cooled, what is the best way of making I hot again?
The temperature of a charcoal cooker is controlled by how much oxygen you allow to get to the fuel. I don't know what kind of grill you have, but typically there are one or more vents at the bottom, and one or more at the top. The bottom vents regulate oxygen supply to the fuel, while the top vents regulate air flow.
If you close down all the vents, the coals will eventually extinguish. If they are still holding enough heat, opening them back up will cause them to reignite. To regulate your temperature, open/close your supply vents (bottom ones) to varying degrees.
Also, consider setting up a zone on your grill surface where there is no charcoal underneath, to where you can move any food in danger of being overcooked.
There are temperature regulators you can get for charcoal grills that control the amount of air getting into the grill to maintain a consistent temperature.
Alton Brown used one in the Good Eats episode "Right on Que"; it looks like this is the model he used: http://www.amazon.com/IQ110-Temperature-Regulator-Standard-Adapter/dp/B00A7F1B60, but I am sure there are others.
Lacking something like this; your best bet is to set up multiple grilling zones. Organize the coals into multiple areas (one with lots of coals that is hot, and one with a lesser amount that is cooler) so that you can move the food between hotter and cooler zones as needed.