I run into these types often. I'm always looking to learn new things so I listen and sometimes it's clear right off that they're loons. I just look serious and listen and if they are incorrect, I point out the errors, gently, and if they are braggers, I just tell them I'm busy and walk away.
I work with a fella like that. Brags on his huge gun collection, his shooting prowess, his skills with the medieval broadsword (I'm not kidding on that one), his high school football career, his gorgeous wife who cooks better than Paula Dean and Rachel Ray, his awesome car, etc. You've been to the moon, this cat's been to Mars. Says it twice, believes it himself sort.
Ah, they're everywhere.