are you also using Honey Pots validation?
this is where you add a hidden form field to the page and validate that it is empty.
add 1 or 2 hidden form elements to the page, then edit the server validations server behavior. set the validation type to Regular Expression, click the lightning bolt next to the server variable and select the hidden element.
set the regular expression to:
and set it to allow blank entry.
Most bots will fill in all form elements regardless of whether they are hidden or not.