When restoring a backup, how do I disconnect all active connections?

You want to set your db to single user mode, do the restore, then set it back to multiuser:

ALTER DATABASE YourDB
SET SINGLE_USER WITH
ROLLBACK AFTER 60 --this will give your current connections 60 seconds to complete

--Do Actual Restore
RESTORE DATABASE YourDB
FROM DISK = 'D:\BackUp\YourBaackUpFile.bak'
WITH MOVE 'YourMDFLogicalName' TO 'D:\Data\YourMDFFile.mdf',
MOVE 'YourLDFLogicalName' TO 'D:\Data\YourLDFFile.ldf'

/*If there is no error in statement before database will be in multiuser
mode.  If error occurs please execute following command it will convert
database in multi user.*/
ALTER DATABASE YourDB SET MULTI_USER
GO

Reference : Pinal Dave (http://blog.SQLAuthority.com)

Official reference: https://msdn.microsoft.com/en-us/library/ms345598.aspx