Added in 1.7

/aclear [-agmnqstu]

Clears the buffer and Channelbar/Sidebar unread color of the specified window type, if no type is specified, all windows on current connection is cleared.

Switches

-a = all windows on all connections
-g - TODO
-m = TODO
-n = channel windows
-q = query windows
-s = status windows
-t = dcc chat windows
-u = custom windows