never mind, when looking through the source I found out that this feature had already been implemented.

r803 | marianoguerra | 2007-08-21 05:40:09 +0200 (Tue, 21 Aug 2007) | 1 line
added hidden option to avoid sending is typing message
sendtyping=0 in the config file disables the typing message.
I think the way it is now, as a config file option, is a good solution. There is no need to have a gui option for that.
But the bug with the preference window opening offscreen remains, maybe that can be fixed
