To clear your history:
Click Tools > Options, and then click History. For Mac, click Preferences > Options, and then click History.
Select the number of days to keep recent messages.
Click Delete History.
This clears all messages up to the number of days selected. You can only clear history up to one day old.
To totally clear your history, you need to delete the history files. To determine where your history files are stored, click Tools > Options, and then click History. For Mac, click Preferences > Options, and then click History. The history file location is stored in the Store conversations locally field.
You can change the history file location. For more information about moving the history file, see Changing the History Files Location.