Pretty self explanatory So like in skype, once you've posted a message, you'd have the ability to click on it and modify what was originally in it (so you can correct things like spelling mistakes or miss information).
Or you could choose to outright delete a message, so that the log of it being posted was still there but it was marked as 'deleted by...'
The chat system was built to support doing this, it has just not been implemented as of yet, due to the way that messages are sent down to the client. It is not a major change however it would require more thought on our part before we implemented it.
We have implemented the editing of a message, and will add the deletion in with the next release.
Deleting an entry has now been implemented, it will be in the next release.