Change from Single User to Multi User

Database is already open and can only have one user at a time.

You should try first :

ALTER DATABASE DBNAMESET MULTI_USER;GO

You can find sessions and kill session

use master
GO

select d.name, d.dbid, spid, login_time, nt_domain, nt_username, loginame
from sysprocesses p inner join sysdatabases d on p.dbid = d.dbid
where d.name = 'db_name'
GO
-- session kill with spid
kill 52