After fiercely pursuing and fantasizing about a potential love interest, bud zoning is the act of relegating said fantasy into a "friend" and no longer being able to picture them as an object of sexual desire.
Bud zoned is the word to refer to a guy that is no longer a sexual interest to a female pursuant.
Friend: "How did your date go?"
Tina: "I can't picture kissing him!"
*Friends make eye contact."
Friend: "Bud zoned!"