Someone "boarding up the closet with rainbow corpses" is blatantly homosexual man or woman who tries to hide their gayness through extreme and sometimes violent homophobia rather than coming out of the closet and accepting their sexuality. Derived from the phrase "coming out of the closet" which refers to admitting to the world that you're gay, and to the fact that the rainbow is usually a symbol associated with gayness.
me:"I can't believe Chris! How can he call me a faggot and then threaten to rape me? Isn't that like, the most homosexual threat you can make?"
Seamus:"Dude, chill, he's just boarding up the closet with rainbow corpses. He's more of a fag then you'll ever be."
(editors note:this is based on a real conversation that occured a few times)