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// Add your own function to filter the fields
add_filter( 'submit_donation_form_fields', 'remove_submit_donation_form_fields' );

// This is your function which takes the fields, modifies them, and returns them
// You can see the fields which can be changed in: includes/forms/class-wpdonations-form-submit-donation.php
function remove_submit_donation_form_fields( $fields ) {

    // Here we remove one of the donation fields (donation_campaign)
    unset($fields['donation']['donation_campaign']);

    // And return the modified fields
    return $fields;
}