How to prevent spam submissions?

Hello is there a way to ban an IP or to block a stalker that wrote me several time a day? thanks

You can use the Unique Submission or Unique Question features to prevent duplicate/spam submissions. 

How to Set Unique Submissions on a Form

How to set Form Limits Based on a Unique Question

You can also add a captcha field to your form. 

How to Add a Captcha Field

Another option is to add Get Visitor Location widget to your form, set it to get the IP and add a condition to your form to hide all form fields is the widget field contains that IP address.

How to Show or Hide Fields Base on User's Answer