Using (declare (special ...))

declare-special.lisp

(defun get-my-value ()
      (list val))

(defun my-value ()
  (let ((val nil))
    (locally
	(declare (special val))
      (setf val t)
      (get-my-value))))

(makunbound 'val)
(unintern (find-symbol "VAL"))

;; (find-symbol "VAL")
;; => NIL
;;
;; (my-value)
;; (T)
;;
;; (find-symbol "VAL")
;; => NIL