I am sorry, there is currently no workaround for that.
It might be possible to add this feature since uddeIM already saves the id of the original message (replyid). A reliable implementation has to check if the sender of the original message is identical to the recipient of the replied message. A not so reliable implementation could allow replies in general. Both implementation will require several changes which may break functionality (e.g. autoresponder, autoforward etc.).
Please create a feature request on the tracker on Joomlacode. I will check if this feature can be added in future versions.
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer