If you use the Survey tool you can configure whether or not anonymous users are allowed to answer the questions, but currently this isn't available in the Form Builder. So you could switch to using this for your forms.

(Unfortunately, while you can rename the app from "Surveys" to "Forms", it will still have buttons like "Answer Survey" which is a bit annoying, and something I've been meaning to change, as the Surveys app could potentially be much more useful than the Form Builder!)

I have it on my todo list to add an "I'm not a robot" type captcha to anonymous forms, I'll bring this forward if it's a siginficant problem!

I'm currently working on setting up a new hosting environment for Voice with more up to date and faster server software, and part of this will include a bit more spammer blocking, which could help a bit. It won't be ready for a few weeks yet though.

I could also look at adding some validation options for form fields (such as email address, postcode, min/max length of answer) as these can help prevent spammers.

Thanks

Joe