Here is an example for a /slap command.

    (defun erc-cmd-SLAP (&rest nick)
      (erc-send-action (erc-default-target)
                       (concat "slaps " (car nick) " around the solar system "
                               "-- just out of spite!")))

And here is an eXtended slap, it slaps repeatedly all of the given nicks.

    (defun erc-cmd-XLAP (&rest nicks)
      (dolist (nick nicks)
        (erc-cmd-SLAP nick)))

Some black magic voodoo will fix the nick completion for /xlap, simply by adding the following defun:

    (defun pcomplete/erc-mode/XLAP ()
      (while (pcomplete-here (pcomplete-erc-nicks))))