Converting the file to MDE will prevent users from altering database objects (design), but not from modifying data.
If you want to restrict what users can do with the data, you could use user-level security.
Note that user-level security is not available in databases that use some of the new file formats (for example .accdb) in Access 2007, so if you convert your database to one of these formats in the future, all configured user-level security will disappear.
Take a look at this guide:
Securing and Administering Access