On every website, contact forms are always there. Lot of spammers comes everyday to spam the website. They burst your database and email limits, sending lot of xxx content in the description. We also face the same in our website.
From last 3 months, we change some code in the website, implement captcha and hurrey!!! It minimizes the spammers up to 90 percent. This is the best way I have ever seen to minimize the spamming on contact forms.
You can use Google Captcha services to restrict multiple form submits from spammers. Google captcha always ask for image verification from humans. If even some bot is trying to submit the form then it will block them.
Self implemented Captcha:
Modern PHP frameworks like Laravel, Yii2, CodeIgniter having inbuilt facility to generate captcha images and verify the code on server-side. If you are using such frameworks then you can easily achieve the Captcha facility. If you are not using framework then you can create your own script to generate Captcha image and verify the code.
Even after implementing the captcha, humans can submit the contact form with captcha content. But after some attempts, they are getting bored and stop sending spam content. It will surely decrease spams by 80 to 90 percent.