There is an easy way to stop spambots (automatic spam submissions) from posting rubbish into your forms and that's to enable the option in your form settings called "Anti-spam honeypot" we have found it very effective and reduces spam coming through forms by 99-100%
To enable this firstly hover over the form you want to enable it for and then go to > 1) settings > 2) form settings
When in the form settings scroll to the bottom and put a tick in "Enable anti-spam honeypot" and then click the "Update form settings" button
and that's it, your now safe from form spam