First, execute below command to see all active processes.
SHOW PROCESSLIST;
Use below command to kill a specific process.
Syntax:
KILL id;
Example
KILL 5;
Use below command to kill all processes.
SELECT CONCAT('KILL ',id,';') FROM information_schema.processlist WHERE user='root' INTO OUTFILE '/tmp/processlist.txt';
Use below command to kill all processes of a user.
kill USER username;