This command is used to cancel the last memo that you have previously sent out. You must issue this command prior to the memo being read, or the command becomes unavailable for that memo permanently. You cannot cancel more than the last memo you sent to a specific user or channel. For instance, if you sent nick1 two memos, you may only cancel the most recently sent one, even if neither one has been read yet. This command accepts 1 argument, a nickname or channel name to cancel memo sending for. To cancel the last memo you sent to #channel, issue the following command:
/msg MemoServ cancel #channel
Note that as far as cancelling the last memo is concerned, this is on a channel by channel or nickname by nickname basis. You are not restricted to cancelling the absolute last memo you ever sent.