Open form results in solicitation & porn

We were testing an idea on this page. Its a wp form which the user needs to enter.

But nowadays I am getting people who simply use it to send porn or random marketing emails. People are using the form to sell their services! which is ingenious ...in a sense...but preety much a short in the dark.

One way is to add a captcha-layer to stop bots. Any oher security measures, you would like to suggest.

https://www.thinkevolveconsulting.com/wishing-you-a-happy-new-year/