blacklist worked....

Well it looks like my blacklist worked last night and the night before. Each night, about 8pm I get about 200 hits from about 20 different ip address's (unfortunately dialups so they change) trying to promote various sick web pages. The past two nights they have been blocked by blacklist. However i've not made any changes apart from adding captcha and the log doesn't say it was denied due to that. What is interesting is that I thought the blacklist would automatically ban ip address's on repeated spams but it doesn't seem to have done that. More investigation needed I think!