I'm not sure I understand this. The autovoice system tries to voice someone only if you're an OP in the channel. If you're using /avoice, that message shouldn't appear. Therefore, I don't know how you're getting that message, but it's not through /avoice itself. So, if it's not through /avoice itself, there must be a script involved, which is the one to blame. Correct me if I'm wrong.
That is an IRC message (coming from the server, I mean). That's a reason for not being able to turn it off: it's not up to mIRC to decide if it's going to be shown or not, it's a server message. Also, one would probably want to know why a command (s)he sent to the server (MODE #chan +v nick) doesn't work.
If you don't want to receive it again, just halt its RAW numeric (482, if I'm right).
If the code above doesn't work, try this one:
RAW *:* You're not channel operator:{ halt }
