Be wary of trying every single security plugin under the sun, as this slows down your site and adds to your maintenance workload by having more things to update and go through if something breaks. At first, stick to the highest rated ones that have proven themselves over time with a lot of users. One such plugin, which is highly recommended by many BuddyPress and WordPress users, is Bad Bahavior. This plugin will not only block a lot of spam, but will make your site invisible to many bots in the first place and thereby prevent fake registrations.