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))))