Making a Sql database read-only

I am trying to configure a database as "Read Only"

If i execute the following query, i get a message saying that this query cannot be completed because there are users using this database eventhough I have no users access this database.

sp_dboption 'my_db','Read Only','True'

So, if I try to make my database offline by

sp_dboption 'my_db','offline','True'

I still get the message saying that one or more users accessing the database when I am absolutely sure that there are no users accessing this database.

What is the problem??

Thanks
Zack