Syntax:
KILL [CONNECTION | QUERY] processlist_id

Each connection to mysqld runs in a separate thread. You can kill a
thread with the KILL processlist_id statement.

Thread processlist identifiers can be determined from the ID column of
the INFORMATION_SCHEMA.PROCESSLIST table, the Id column of SHOW
PROCESSLIST output, and the PROCESSLIST_ID column of the Performance
Schema threads table. The value for the current thread is returned by
the CONNECTION_ID() function.

KILL permits an optional CONNECTION or QUERY modifier:

o KILL CONNECTION is the same as KILL with no modifier: It terminates
  the connection associated with the given processlist_id, after
  terminating any statement the connection is executing.

o KILL QUERY terminates the statement the connection is currently
  executing, but leaves the connection itself intact.

If you have the PROCESS privilege, you can see all threads. If you have
the SUPER privilege, you can kill all threads and statements.
Otherwise, you can see and kill only your own threads and statements.

You can also use the mysqladmin processlist and mysqladmin kill
commands to examine and kill threads.

*Note*: You cannot use KILL with the Embedded MySQL Server library
because the embedded server merely runs inside the threads of the host
application. It does not create any connection threads of its own.

URL: http://dev.mysql.com/doc/refman/5.5/en/kill.html

