You should not give up, eventually someone will find a solution for this. I got the same problem.
My hosting closed down my forum this morning. Other phpBB sites I know off are being atakked similarly, and the directory name does not seem to do any good, mine is called "foros". Registration is not a solution, they do not register. Ip banning is not working either, they come with different ip's (up to 300 different ips)
They don't do anything, the just lurk around, till your forums goes down or the server crashes, then they start again.
It' seems to be popularity and good google indexing what gets them on your back.
I've seen several posts already about this problem here on phpBB.com but no body seems to be taking it seriously.
is there any way to stop all this thousand of lurkers?