You can always limit who can Self-Book Events on your calendar by controlling who you share your calendar with. To do this, you can create a private share list for your calendar that is limited to your approved Keep&Share users. If you would like to privately share your Calendar with someone who is not a current Keep&Share user, you can use the “Invite-a-Friend” option to set up a free account for them or if you have a paid Keep&Share account you can send the Private Sharing Link.
If you choose you to share your calendar publicly, there is a captcha that is automatically enabled on your Self-Book forms. This is to protect your forms from spam or bots automatically filling them out.