SQL Enterprise Manager provides an option to shrink a database. This is done by internally calling DBCC SHRINKDB. Note that the SQL Server must be started in single-user mode to perform these operations.
If it's not started in single-user mode, there will be users, system handles such as the CHECKPOINT and MIRROR handlers, that can't be removed from the database and, therefore, prevent the operation from succeeding.
To start SQL Server in single-user mode use the -m keyword and start SQL Server from the command-line.
Check for more info http://docs.rinet.ru/MSSSSSSQL