If you are referring to Formidable there is a great article for reducing spam emails that come from the forms.

https://formidablepro.com/knowledgebase/add-spam-protection/

If it's from the Booked Plugin, then just install Really Simple Captcha and activate it. 

https://wordpress.org/plugins/really-simple-captcha/

If this is from WP Booking System, make sure you have Captcha enabled under WP Booking / Forms /

Cheers!