How to close the possibility of spamming through the registration form?

Question

Joomla

There is a site on Joomla, someone started spamming various emails through the registration form. I don't really understand the spammer's logic, in the registration form there is only an email and a password, after that a confirmation link comes. Everything.
The form is not protected at all. Didn't bother. But I get about 10 undeliverable email reports every day.
How can you protect yourself from such spam?

Answer 1

Captcha or honeypot.

Answer 2

The simplest and without captcha: replacing the url handler through js at the time of submitting the form.
If spam does not stop, then you can turn on the captcha.

Answer 3

I don't really understand the spammer's logic

means a lot of text fits into the form)