When you want to rename a database or something like that you need to get exclusive access to the database and it can be hard to get this exclusive access because there are always someone who has a connection. This script will kill all connections to a database, just replace "DATABASE_NAME" with the name of the database you want to kill connections for,

DECLARE @SQL VARCHAR(8000)
 
SELECT @SQL=COALESCE(@SQL,'')+'Kill '+CAST(spid AS VARCHAR(10))+ '; ' 
FROM sys.sysprocesses 
WHERE DBID=DB_ID('DATABASE_NAME')
 
EXEC(@SQL)