Im having a terrible problem with people joining and assaulting the forum! HELP!

[Desu]

Theres people who keep joining my forum, and posting pornographic images, and images of people doing disgusting and disturbing bodily things. They're using something that keeps changing their IP address, repetitively. They've done this a few months back, and we just got rid of all of their posts, and they hadn't shown. But they registered earlier in the day, and waited till all of the staff was offline, and began to attack the site. They wont stop. I IP ban and delete the accounts, but they keep finding ways back in! Please help............its destroying the forum.

[KingOfSports]

Make it so that no new members can join or so that admins must accept any new members.