One from the console login as root, change to the /var/log directory. Then use the rm command (rm -f logfiletodelete). This command breaks down like this
rm - Remove -f - remove with out asking "are you sure" logfiletodelete - the name of the log file.
Second login as loginuser using ssh then su to root and delete files.
Third in version 3.040 and above use the WebAdmin GUI to delete old log files