As root do;
service cups stop
rm -r /var/spool/cups
service cups start
You can if desired save the above three commands as a script file and make the file excutable. Example the filename is ClearCups and the file has the three lines within, when done saving the file to root's bin directory as root do; chown +x ClearCups
Then all you have to in the future is type in the script file name; ClearCups
You can also make this a cron job which runs daily, weekly or?????