You may do it with sqliteman.
You can install sqliteman from the Ubuntu Software Centre.
First, not just close but really quit Skype (right click on the Skype icon in the Unity panel and select quit).
Now make a backup of the Skype sqlite database. With your favorite file manager go to $HOME/.skype/YOUR-SYKPE-NAME/
and duplicate the main.db
file.
Start sqliteman and open the file $HOME/.skype/YOUR-SYKPE-NAME/main.db
You may need enable displaying of hidden files: Do a right click in the file chooser somewhere in the filelist and check "display hidden files".
When opened, on the right site at the top you see the SQL Editor. Run the following SQL-commands, one by one by clicking the single green arrow at the top. After each run clear the SQL-Editor and paste/edit the next one.
Replace SKYPE_NAME
with the real Skype name of the Skype partner, not with an alias name!
DELETE FROM messages WHERE
dialog_partner="SKYPE_NAME" OR
chatname LIKE "%/$SKYPE_NAME;%" OR
chatname LIKE "#SKYPE_NAME/$%"
DELETE FROM chats WHERE
dialog_partner="SKYPE_NAME" OR
name LIKE "%/$SKYPE_NAME;%" OR
name LIKE "#SKYPE_NAME/$%"
DELETE FROM chatmembers WHERE
identity = "SKYPE_NAME" OR
chatname LIKE "%/$SKYPE_NAME;%" OR
chatname LIKE "#SKYPE_NAME/$%"
DELETE FROM participants WHERE identity="SKYPE_NAME"
DELETE FROM conversations WHERE
identity LIKE "%$SKYPE_NAME;%" OR
identity = "SKYPE_NAME" OR
identity LIKE "#SKYPE_NAME/$%"
DELETE FROM transfers WHERE partner_handle = "SKYPE_NAME"
DELETE FROM voicemails WHERE partner_handle = "SKYPE_NAME"
Close sqliteman (if asked, you don't need to save changes of the SQL editor) and start Skype. The history of SKYPE_NAME
should be gone.