That wont stop them, if they want to become a member and post it anyway unless there is a stand down when they cant post for say seven days. Maybe stop them posting links might help.
Apart from anything else, if we allow guest posting it means that members who forget to log in end up posting as guests. That gets very confusing, very quickly.
At least if the spammers have to sign up before posting it slows them down a bit.
I've now corrected the settings on the new forums to be the same as was already the case with the General Discussion forums, so there is no guest posting and there is a moderator who can kill any spam which might be posted by registered users.
I've also deleted a few more obviously bogus users.