How can this form spamming be stopped?

Find the IP address of the poster in your ACP and ban it.

Also, look into a Cleantalk.org subscription. Best $16 a year you'll spend.

I hardly get spammers with active IPS subscription - it does good enough job for me. 

For other boards  - i use captcha, ban temp email address and spam bot - ASN's or Subnet if from datacenter or proxy location. 

It is worth switching to using hCaptcha, which we find is the most effective at preventing spam registrations. You can set the sensitivity level on this also.

I have got CleanTalk and went to hcaptcha. It has helped tremendously. Blocked over 2,000 post in 24 hours. Some are still getting through but it is way more manageable for now.

Glad to hear that resolved your issue 🙂