Form spam is an unfortunate fact of life for most nowadays.
Many people use some form of Captcha to limit this & there are a host plugins available to do this – see https://ww.wp.xz.cn/plugins/search.php?q=captcha
A more modern approach is to use a Honeypot method which tends to very effective against Bots. Again there are a host plugins available to do this – see https://ww.wp.xz.cn/plugins/search.php?q=honeypot
If you know the sender email address, why not simply use Comment Blacklist?
CAPTCHA also consumes CPU much. Honeypot might work but only 50/50 possibility.
But the best way to block them is detecting the source IP address and blocking accesses from the address with firewall. Ask your host support if you can use firewall on the host.
How can I find out that email IP address? I inspected headers but I was unable to do that.
