Delete Slow Query Log File
As of 5.5.3, FLUSH LOGS will close and reopen the slowlog.  (In old versions, FLUSH had no effect on the slowlog.)

So, on *nix OS, this should work without restarting the server:


rm (to delete) or mv the slowlog to another name.  (Note: mysqld will continue to write to the file, even though you changed the name.)
FLUSH LOGS;.  As of 5.5.3, you can limit the effect via FLUSH SLOW LOGS;


See http://bugs.mysql.com/bug.php?id=14104 and http://bugs.mysql.com/bug.php?id=60878

I would not turn off the slowlog -- next month someone will add a naughty query and you will want to know about it.