>> How can I know how many current users for the database?

When you connect to a database, SQL Server aquires a shared lock on the database so that it can keep track of which connections are using which database.

Execute sp_lock and look at the results. You should see a shared locks (MODE=S) for the database (TYPE=DB) and the dbid.

>> How can I don't let users access this database?

Take a look at sp_dboption. Specifically the "single user" or "dbo use only" options. You're probably most interested in "dbo use only."

-Mike