How to secure my form from spammer?

2) Recently a spammer used this page to send spam email and I want to secure it so that it will not happen. Please advise how I do this.

Hello,

You can prevent spamming by changing your form submissions to Unique Submissions. Additionally you may find it useful to add a Captcha field to your forms. 

Here is the related user guides:

How to Set Unique Submissions on a Form

How to Add a Captcha

If you need further assistance please let us know.