The pos argument can specify the  environment from which to remove
  the objects in any of several ways:
  as an integer (the position in the search list); as
  the character string name of an element in the search list; or as an
  environment (including using sys.frame to
  access the currently active function calls).
  The envir argument is an alternative way to specify an
  environment, but is primarily there for back compatibility.
It is not allowed to remove variables from the base environment and
  base namespace, nor from any environment which is locked (see
  lockEnvironment).
Earlier versions of R incorrectly claimed that supplying a character
  vector in … removed the objects named in the character
  vector, but it removed the character vector.  Use the list
  argument to specify objects via a character vector.