Is there a word to describe an individual who has complete control over his negative and positive emotions?

Equanimous

from Wikipedia: "(Latin: æquanimitas having an even mind; aequus even animus mind/soul) is a state of psychological stability and composure which is undisturbed by experience of or exposure to emotions, pain, or other phenomena that may cause others to lose the balance of their mind."

There are several appropriate terms. Assuming that all emotions could be described as positive or negative(is there such a thing as a neutral emotion?)

Stoic In the classical sense this meant adherents to the stoic philosophy which was a philosophy emphasizing the importance of the appropriate behaviour, in modern usage it would be used to describe someones demeanor.

"Angela Merkel is of very stoic character"

Similar terms would be "unflappable" and "impassive".