the message, which function is found in the Message menu (not right click).
... the concept of cancelling a message opens another can of worms.
The idea would be that -1- you send a message in error -2- that message
reaches the news server and propagates to other news servers all over
the world -3- you then wish to 'unsend' the sent message, so -4- you
send out a cancel message, which theoretically would first go to the
news server you sent to in the first place, and then -5- that news
server would propagate your cancel message to all of the other news
servers all over the world which would then -6- honor your cancel and
cancel your original message.
The can of worms is that in its simplicity, the cancel message function
lends itself to 'rogue' (or bogus) cancel messages which are initiated
by someone other than the original message sender.
Consequent to mitigating the problem of rogue cancels, typical news
agent developers have chosen to eliminate the ability to cancel a
message (send a cancel message message) from the agent. And further,
typical news server admins have chosen to eliminate the 'vanilla' cancel
message function of the news server. That is some admins
eliminate/decline/refuse to cancel a message, but they will propagate a
cancel message to let other admins do with it whatever; or some other
admins will not cancel and not propagate the cancel.
... along comes the concept of cancel lock and cancel key, which enables
a security process which prevents anyone but the sender of the message
from cancelling a message. This allows certain servers with such a
process in place to be able to honor cancels and also to propagate cancels.
This is the case for such as news.individual.net and eternal-september
which honor and propagate cancel messages which were initiated by their
clients and the clients of the other servers with cancel-lock-key.
So; you actually /can/ cancel a message, in a limited sense.