This is a query I use often to check what active connections exist for a specified database:
select spid, sp.cmd, sp.hostname, sp.loginame, sp.nt_domain, sp.nt_username, sp.program_name from master.dbo.sysprocesses sp where db_name(dbid) = 'mydatabase' and DBID <> 0 and spid <> @@spid
If I want to take the database offline I’ll then kill these processes using the spid.
I also found a great post today that had a nice way of killing ALL active connections, by running the following sql:
alter database dbName set single_user with rollback immediate
This can then be reverted out with
alter database dbName set multi_user with rollback immediate