How would you know if a user is accessing the schema in question? What if the user is mainly accessing a different schema? You want to kill that user's session?

What do you mean by using a schema? Performing one query against a table in that schema? Continuously running scripts? Being the schema owner and being killed because you are accessing your own schema?

If you want to kill a session:
alter system kill session 'sid,serial#';

You get sid and serial# from v$session, and they go inside single quotes in the alter statement.