If you only have one form, it's relatively easy. More than one form, gets trickier.
You can search this forum for "securing a database" for lots of details.
In summary, you need to hide the ribbon, disable the Access "special" keys that let you bypass the startup sequence, define a startup form, and put code in that form such that if you try to leave the form, it quits the application.
For these topics, search for:
hide ribbon
node navigation pane
define startup form
disable bypass key
close application from event code
IF you have more than one form, then you create what is called a "switchboard" and make it ALWAYS present via the methods discussed above. Then you create other forms to be launched by command buttons that perform the "OpenForm" operation. (There is a command button wizard option for that function if the targeted form already exists.)
If you are using a switchboard or dispatcher form then you don't care if the user exits the forms loaded via command button. You only care that the switchboard stays in place like a blocker. And the only "Exit app on Close" trap is on the switchboard in that case.