Oracle SQL editor tools

Try the following:

select s.username, s.sid, p.spid 
  from v$session s join v$process p on addr=paddr
where s.username = 'USER2';

Using the spid value from the above query, log on to the server and issue the following command from a DOS command prompt:

orakill YOURSID spid

YOURSID is the SID of the database instance you're dealing with.

orakill often works when the GUI tools fail to kill processes. Here is a nice overview of orakill. Note reason #1 for using orakill - it might explain why you can't use your GUI tool:

The alter system statement will not clear any locks that exist. Instead, the session will remain connected until it times out, then the session is killed and the locks are released. The orakill command will kill the thread and the locks instantly.

Update:

You can also try:

select sid, serial#, program from v$session;
alter system kill session '<SID>,<SERIAL#>' immediate;

although I wouldn't hold out a lot of hope...

I don't know if Enterprise Manager 11g looks the same as Enterprise Manager Grid Control 10g, which is what I use. If it's similar, there should be a Security page, where you can manage users, roles, privileges, etc. Failing that, for sure you could use the following SQL commands:

CREATE USER xxx IDENTIFIED BY pppp DEFAULT TABLESPACE users;
GRANT CONNECT, RESOURCE TO xxx;

GRANT privilege TO xxx;   -- repeat for each privilege in your step 4.