init-elpa.el

(defvar package-filter-function nil
  "Optional predicate function used to internally filter packages used by package.el.

The function is called with the arguments PACKAGE VERSION ARCHIVE, where
PACKAGE is a symbol, VERSION is a vector as produced by `version-to-list', and
ARCHIVE is the string name of the package archive.")

(defadvice package--add-to-archive-contents
  (around filter-packages (package archive) activate)
  "Add filtering of available packages using `package-filter-function', if non-nil."
  (when (or (null package-filter-function)
            (funcall package-filter-function
                     (car package)
                     (package-desc-vers (cdr package))
                     archive))
    ad-do-it))



;; Only take certain packages from Melpa

(setq package-filter-function
      (lambda (package version archive)
        (or (not (string-equal archive "melpa"))
            (memq package '(magit rvm slime)))))