Ladilla is the spanish word for crab louse. Therefore it's usually used to reflect that someone can be as annoying as a crab louse. It can also be use in the form of a verb "ladillar". For it's sexually related content, it can also be found in expressions with such intention.
"Hombre, eres una ladilla" as in "Boy, you are such a pain", or "deja de ladillar" as in "Quit fooling around".