meaning Persistant Little Bitch.
aka: someone who doesn't give up until their point has been made, or until they get what they want. Usually someone who is very stubborn.
Melissa: (adds an unflattering picture of Rebecca on facebook)
Melissa: Nooo! It's so funny!
Rebecca: MELISSA DELETE THIS NOW. Am I going to have to be a PLB on your ass?
"Don't be such a PLB!"