It seems that the plugin you suggested can work alongside the reCAPTCHA, because you don't need to switch to it in Global Configuration. It seems to work on another website which uses the standard contact form of Joomla only, but in the list of components to activate it for, Helpdesk Pro is not listed so I'm not sure it will work there.
Yes, I understand that. But the plugin you recommended does not appear in the list of captcha's in Global Configuration, only the two standard of google appear.
The plugin does seem to work in general, because the standard contact extension of Joomla (on another site of mine) immediately uses it (meaning that it uses two: that plugin, and the Google CAPTCHA that is selected in Global Configuration). But the link to the plugin's site does not show on the Helpdesk Pro form page.
I don't know if it will affect Helpdesk Pro too then or not, but it is installed and activated, and the Invisible CAPTCHA of Google is active too. So I will see if spam still comes in now or not. So far, I got nothing yet.
I have the same problem since some days ago. Why there is no text filter to easy detect some spam ? we could choose words in configuration as viagra, bitcoin, ...
Or there is the filter but I don't find it?
Same problem with standard google recaptcha and invisible recaptcha.