As simple as select + delete.
In the user’s folder within home folder press ctrl+h
then inside .Skype folder enter the folder with the skype user name you want to clear history for and delete all the files with an extension ‘.dbb’ .
the terminal way of doing it, say the user’s skype username is ‘rome’ and the computer’s name is ‘russia’ :
rome@russia:~$ cd /home/rome/.Skype/rome/
rome@russia:~$ rm -rf `find . -name ‘*.dbb’`
and you’re done.