You aren't using the standard Net Send command line function. You are using some third party software to manage it. Uninstall it or stop using it and the messages will go away.

Now, of course, if you have been sending suggestive messages to "dirty Gertie" in Receiving and she is pitching a bitch fit and you can't find a way to uninstall the application then you have a teeny problem......

Then again, since the popup will show in Gertie's event log if logging is turned on then you have a bigger problem still......