Pull the legs off a spider 

When A is making fun of B in front of everyone, and C, who's B's friend says "Hey, go pull the legs off a spider.", to A, C is telling A to find something better to do with their time than to make fun and laugh at C.

B is the one being laughed at by A.
C, in saying this to A, is doing several things at the same time.
Showing strong support for B.
Showing clearly that A's laughter is entirely inappropriate.
Also saying that A is the type of person who'd enjoy ruthlessly pulling the legs off a spider, is not only a bully but also acts like a child.

"Hey Dave are you a total fag or something?", said Dan, as everyone in the room was laughing at Dave, including Dan. Then Dale, sticking up for Dave, said to Dan, "Hey, go pull the legs off a spider." Then Dan felling small and belittled stopped making fun of Dave.

Pull the legs off a spider by Rusty Shacklefurd December 20, 2007

Pull all the legs off a spider 

When a male masturbates 8 times in a day.

Jamie: During my day off yesterday I was so bored and sexually frustrated I had to pull all the legs off a spider to keep me from molesting my dog.

Pull all the legs off a spider by howlingmadwilger October 29, 2010