How can I prevent spam on my custom HTML form?

[awesomeib]

Due to a tricky integration with Salesforce, my form is required to be custom built, so I can't use Jotform, Wufoo, etc. to stop the spam. I need to know how to do it with a custom HTML form. I tried reCaptcha, but it didn't help at all. Maybe there was something I did wrong. How can I stop these spam submissions from coming through?