I have been using contact form 7 on many websites for the past few years. I have noticed that there are some anti spam measures which are being defeated by spam bots (computers that post spam into forms).
I am reluctant to use Google Captcha because I think that while it is undoubtedly effective, it also presents a barrier to some users, particularly elderly ones. I have also seen preferential treatment (aka easy access) when a user is using Chrome, but other browsers they may be stuck answering questions makes bypassing it frustrating to say the least.
I have just found Contact Form 7 Image Captcha by KC Computing (Kyle Charlton) which is very elegant, simple and works fine. It asks the user to click on an image which matches some text. A user has to complete this step before submitting the form.
If your site is using Contact Form 7, you can load this plugin into the library and then insert the following shortcode into the form: [cf7ic “toggle”]
If your website uses the form in a dark coloured panel, the formatting of the text may not work very well. To get around that problem you can add the following text into the CSS for your site. For Divi users this can be found on the Divi settings page at the bottom. There is a space to add CSS in there. It will change the selected text to yellow.
.captcha-image .cf7ic_instructions span {
color:yellow !important;
}