When People forms are shared publicly, there's no way to prevent someone with malicious intent from submitting a form; however, you can block suspicious email addresses from making future submissions by adding them to the spam list.
Note
Profiles created by spam form submissions must be deleted manually.
-
Select the gear icon on the People page, then choose Manage spam list from the dropdown.
-
Add the email address that you would like to block, and select Mark as spam.
Once an email is marked as spam, a spam icon shows next to it on the associated profile.