How to get rid of spam?

Question

Fight against spam

I have several sites on the same site. All forms have passive protection, a hidden field that should remain empty, in the name field it is allowed to enter only Russian letters, another field with a minimum number of characters to be entered. The strangest thing is that the spam emails that come in have fewer characters than allowed in the character-limited field. And spam comes only to those sites that are on http, spam does not come to sites with https. Maybe a coincidence, but rather strange.

Answer 1

the strangest thing is that spam emails that come in have fewer characters than allowed in the field with a limit on the number of characters entered.

Apparently the protection is on the client, but this does not interfere with sending a post request. Make ajax forms and disable cross-domain requests. Well, be sure to implement validation on the server.
In general, validation on the client is not protection, it is user convenience. She gets by without a problem. So if you implement validation as protection, then on the server.

Answer 2

try reCATPCHA, well, or write something of your own.
The technologies of spammers do not stand still, the feint with an empty field has not really saved for a long time.