Preventing spammers from using mail script

Is there a way code a php mail script so that spammers cannot utilize
it to blast spam? At the moment, we are using a hidden value in the
form for validation but anyone can see the hidden value in the form and
push this value to the script. Would posting the form to itself (ie.
embedding the php codes on the same page as the form) help in this
matter? Any tips?

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