Hi,
There are several options available to you assuming that your database:
Change the start-up options: You can change the stat-up options so that the database window is not available to the user. Unfortunately, if the user holds the shift key down when entering the database this action will be cancelled. You could uncheck the hidden objects box (tools, options, view) and hide each database object by right clicking on the object in the database window and going into its properties.
Create an MDE: If you turn the front end of your database into an MDE the user won't be able to make any design changes. However, they'll still be able to open the database objects and therefore alter data directly through the tables.
Create a run-time version: This is the most secure method because the database window cannot be viewed. This also has the added advantage that the user doesn't need to have Microsoft Access installed on there PC if you distribute the whole run-time package.
The problem with creating a run-time version or an MDE is that when you make any design changes to the front-end of the database you will need to re-distribute it.
Hope this helps!