You can abort a connection (close it with an error) with:
void SAP_API RfcAbort(RFC_HANDLE handle, char * text)
Thus this function sends an error message, if possible, and closes the connection.
A given error message cannot be sent if the receiver is not in state where it expects to receive some RFC data.
This function is defined in SAPRFC.H.
Handle to RFC connection. If handle is RFC_HANDLE_NULL, all connections are aborted.
If you supply a text, the text is used as error message on the receiving side. If the text field is NULL, the connection is only closed.