To avoid spammers the site

hi,

i am using express engine 1.6.7.

In the forum , i enabled captcha in member registration and check the option for only when the admin allows the user then only he can allow to post in the forum.

but so many spammers ( persons) using my forum for their advertisements. there are so many spammers coming in bulks everyday.I can’t check each and every registeration and block. is there any easy way to block such spammers. ?

i need to avoid these spammers in any like keywords block and so on etc.,

kindly plz help what are the plugins or modules needs to use to block those spammers.