Save the file as .accde (in save/publish for 2007/10) or .mde for 2003 or earlier
Make sure you compile the db before saving or the save will fail. Also make sure you keep a 'master' copy since you will be unable to decompile the new db.
Note that users will still be able edit and add tables and query designs - to protect against this you will need to do things like hide these objects.
To protect tables put them in a back end if not already done (it is best practice to do this anyway) and password protect it then relink in your front end.
For queries, make sure you have a suitable backup or better ensure all queries are within the recordsource/rowsource etc of a form or hardcoded in vba