I'm sorry for you, but I report what the mIRC help say about it:
The equal sign in =$nick is required to send the reply as a dcc chat message. If no equal sign is used, the message is sent as a private irc server message.Then the =$nick is only to refer a nick in a dcc chat window, only to msg commands purpose.
Instead the mIRC help, about close command says:
/close [-icfgms@] [nick1] ... [nickN]
Closes all windows of the specified type and the specified nicknames. If no nicknames are given, all windows of the specified type are closed. The type of window is denoted by c for chat, f for fserve, g for get, i for inactive dcc windows, m for message (query), s for send, and @ for custom windows.
You can specify the Nth window for -cfgs by appending a number, eg. /close -s4 nick, would close the 4th open dcc send to nick.
You can also use a wildcard as the window name and all matching windows will be closed.As you can see no =$nick is mentioned.
Please, read the help files before you post things like that, thanks