There are a number of things you could do, a checksum is probably
overkill.

You haven't made clear what you are trying to do:

a) Prevent people from accessing the DB between set hours
b) Allow people to access the DB but note that it has happenned.

You can put a database into offline mode using gfix / gsec / gbak (I
forget which one). In offline mode, only SYSDBA and the owner can
connect, so it might be as simple as a script to do that.

Other things like shutting down the firebird service (on a dedicated
server only obviously).

Renaming the database alias is another option providing no-one is
connected at the time you "shutdown" the application.

Creating an embedded Firebird application that steals control of the
FDB file would also prevent others connecting to it.

If you were preventing the fdb from being accessed at all, you could
simply note file changed timestamp from the OS.

Lots of ways to skin this cat.

Adam