Using Recaptcha v2 and Guest Entries plugin
I don't have much experience with these, but here are some potential plugins you can look into for Craft 2 that use various anti-spam techniques:

https://straightupcraft.com/craft-plugins/snaptcha

https://straightupcraft.com/craft-plugins/sprout-invisible-captcha

https://straightupcraft.com/craft-plugins/google-recaptcha

https://straightupcraft.com/craft-plugins/spam-guard