How can we prevent spam submissions?

In addition we have been spammed. Can you please tell me how to add a "capcha" so we can avoid spam ? 

There are two ways you can prevent spam submissions:

1. Add a CAPTCHA field to your form.

Complete guide: How-to-Add-a-Captcha-Field

2. Or, you can enable the UNIQUE SUBMISSION feature which can check the respondent's browser cookies or IP (or both).

Complete guide: How-to-Set-Unique-Submissions-on-a-Form