How to forcefully disconnect a connected user from an Oracle Databases

Find existing sessions to DB using this query:

SELECT s.inst_id,
       s.sid,
       s.serial#,
       p.spid,
       s.username,
       s.program
FROM   gv$session s
       JOIN gv$process p ON p.addr = s.paddr AND p.inst_id = s.inst_id
WHERE  s.type != 'BACKGROUND';

you'll see something like below. 

Then, run below query with values extracted from above results.

ALTER SYSTEM KILL SESSION ',';

Ex: ALTER SYSTEM KILL SESSION '93,943';