Open Question: how to stop recieving spam email from my website?

I have a website and there is a simple contact form in it containing name, phone and email. I recieve 100s of spam email through the contact form, although I validate the form ( so for example, they have to enter a phone number) but I see that in the phone number field they write lines of urls and text. what should I do?
I am using the mail function in the php for my website.
please advise.
thanks