Use of the contact form

I cannot receive emails from people who use my form to send advertising.
How to protect it?

Are you saying you want to or don’t want to receive emails from people who use your form to send advertising?

yes but especially that it cannot use it with a robot … which is the case currently

Any reason why you don’t use the re-captcha in the forms plugin.
That should give you a reasonable protection against robots without much hassle
Of course the better the protection the more complex it is for users.