One of the downsides of having a contact form on your website is that it allows people to spam you. Some of them are pretty cocky about it too, saying that because you have a contact form on your website, that they are entitled to spam you using your form.
As frustrating as this is, please do not mark these emails as spam.
The problem with marking these emails as spam is that the form has come from your website, and is likely using your domain name. If you mark these emails as spam, then you are marking your own domain name (your website address) as spam.
There are processes that can be put in place to try to alleviate the amount of spam a website (usually through comments) and a website owner (through your website forms, and if your website email address is on your website) receive.
Unfortunately spammers seem to be constantly innovating ways to get their message into your email inbox. All we can do is to try and keep up and make it as difficult as possible for them to send that email to us.