I'd suggest you check out this FAQ: https://wpforo.com/community/faq/how-to-stop-spam/

To help prevent spam in the registration form, there are a few great plugins you can use:

1. Akismet: It's a popular anti-spam plugin that works well with wpForo. It automatically filters out spammy registrations and comments. Make sure you have an API key to activate it.

2. reCAPTCHA by Google: Adding a reCAPTCHA to your registration form is a solid way to deter bots. You can use plugins like WP reCAPTCHA Integration or Advanced noCaptcha & Invisible Captcha to implement it easily.

3. Antispam Bee: This is another excellent plugin, especially if you're looking for a solution that doesn't require an external service. It’s lightweight and effective for preventing spam.

4. Spam Protection, AntiSpam, FireWall by CleanTalk: CleanTalk offers a comprehensive solution with spam protection and firewall features. It’s a paid service but provides a great layer of protection.

5. Stop Spammers: This plugin provides a wide range of anti-spam features, including blocking IPs, checking against spam databases, and more. It's a robust option if you're facing persistent spam issues.

For the best results, you might want to use a combination of these plugins. For example, using reCAPTCHA along with Akismet can significantly reduce spam. 

I thought I would try Wpforo for the first time, just wondering what the best plugin for preventing spam in the registration form?.