Can we restrict the number of connections to a database in SQL Server?
Seems like we can do it : 

To configure the user connections option


In Object Explorer, right-click a server and click Properties.  
Click the Connections node.
Under Connections, in the Max number of    concurrent connections
box, type or select a value from 0 through    32767 to set the
maximum number of users that are allowed to connect    simultaneously
to the instance of SQL Server.
Restart SQL Server.


http://technet.microsoft.com/en-us/library/ms187030.aspx