Here are a few suggestions from several sources.
From the Oxford Dictionary:
chatterbox |ˈ ch atərˌbäks|
noun, informal.
A person who talks at length about trivial matters.
From Roget's Thesaurus:
chatterbox. noun, informal.
He was the office chatterbox. He was the office talker, chatterer, prattler. Very informal, windbag, bigmouth, gasbag, blabbermouth, motormouth.
A chatterbox generally brings to mind someone who will simply not stop talking.
There are more formal words that you can use. Verbose is one. However, you cannot say "a verbose" like you can say "a chatterbox" because verbose is an adjective.
From Roget's Thesaurus:
verbose - adjective.
Try not to be so verbose when you're being interviewed. Try not to be so wordy, loquacious, garrulous, talkative, voluble; long-winded, flatulent, lengthy, prolix, tautological, pleonastic, periphrastic, circumlocutory, circuitous, wandering, discursive, digressive, rambling; informal mouthy, gabby, chatty, motormouthed. antonym succinct, laconic.
