To kill a fork bomb that you can't send a kill(-pgid), you need to send
a STOP signal to each of the processes. The fork bomb won't grow past the users process limits and a STOPped process can't fork. So once all the processes are stopped you can KILL them.
Many years ago we had a lot of fun here in an fork bomb arms race. That's where several forms of "comets" mentioned above were invented in an effort to find something that the sys admin (me) could not kill.
One neat way to kill a comet, is to create a fork bomb as the user of the comet! That will slow down the comet enough so you can STOP it. Then you kill the fork bomb in the usual way.