Ways to stop access to a SQL express database
There is no way you can secure a SQL Express database against access from a computer administrator. In the typical SQL Express scenario this means that the user of your application (which is usually a also local administrator) can see everything in the database and it can also modify anything, no matter what grants or permissions you establish. 

If you want to protect the IP in the database (which is the usual reasoning behind this ever repeating question) you need to prevent access to the physical database which you can only do by no releasing the database to start with. In other words, use a web service instead (hosted by you), or a cloud based database like SQL Azure.