They still try to sign up, but none of them can actually bypass the captcha. I implemented a custom callback that returns a HEAD response with 404 to fool them (because I noticed that attacks intensified when I returned 2xx or 3xx responses).
You can take it one step further and check how fast the form was submitted. If it is below the predefined threshold, it is probably a bot.
[0] To hide the form field, you can use one of these snippets, interchangeably:
"display:none;"
"position:absolute!important;top:-9999px;left:-9999px;"
"position:absolute!important;height:1px;width:1px;overflow:hidden;"