The CLOS symbols are EXPORTed from the package “CLOS”. “COMMON-LISP” uses (as in USE-PACKAGE) “CLOS” and EXT:RE-EXPORTs the [ANSI CL standard] standard exported symbols (the CLISP extensions, e.g., those described in Chapter 29, Meta-Object Protocol, are not EXT:RE-EXPORTed). Since the default :USE argument to MAKE-PACKAGE is “COMMON-LISP”, the standard CLOS symbols are normally visible in all user-defined packages. If you do not want them (for example, if you want to use the PCL implementation of CLOS instead of the native one), do the following:

(DEFPACKAGE "CL-NO-CLOS" (:use "CL"))
(DO-EXTERNAL-SYMBOLS (symbol “COMMON-LISP”)
  (SHADOW symbol "CL-NO-CLOS"))
(DO-SYMBOLS (symbol "CL-NO-CLOS")
  (EXPORT symbol "CL-NO-CLOS"))
(IN-PACKAGE "CL-NO-CLOS")
(LOAD "pcl")	; or whatever
(DEFPACKAGE "MY-USER" (:use "CL-NO-CLOS"))
(IN-PACKAGE "MY-USER")
;; your code which uses PCL goes here