how to keep bots from joining

I hate to be pessimistic, but when 3 peeps join our forums with somewhat similar non-names with only the last 4 letters being different, along with a non-word email address…..a red flag goes up.

Anyhow, is there a plugin to add security to joining my bbPress forums or apparently my WP site. My old phpBB forums had the “ask a question” field where someone needed to answer a question to join, is this available in bbPress/WP forums? If not, how do I add typing in an image………sry I don’t know the official names but any assistance is appreciated….thanks!