You can use captcha to avoid spam. The document below will guide you on how to integrate CAPTCHA into your site:

Adding to Contact Form 7

Using reCAPTCHA (recommended)

Step 1: Get the keys

• Go to Google’s reCAPTCHA admin page.
• Register your site

• Get the site key and secret key

Step 2: Enter the key pair into the WordPress admin

• Go to the admin menu Contact > Integration.
• Find reCAPTCHA panel and click Configure Keys.
• Copy the site key and secret key into the appropriate fields and save them.

Step 3: Insert reCAPTCHA tag to your form.

Using Really Simple CAPTCHA

Step 1: Install and activate Really Simple CAPTCHA

Step 2: Insert this code to your form:
Insert Text below [captchac captcha-1][captchar captcha-1 placeholder "Insert here"]

You will see your form same this:

Adding to Gravity Form

Using reCAPTCHA