A quick thought:
If the database is being used purely for storage, put one dummy form in and set it as the startup.
On this form in the Form_Open event put the following code:
MsgBox "This database is used by application X and should not be opened with Microsoft Access. Please use application X for all data access with this database", vbOkOnly+vbCritical
Application.Quit
then when they open the database with access, it will display this message and quit when they hit the OK button. Then disable the AllowByPassKey property to stop people opening it with shift and convert it to an MDE file (Delphi should have no problems with this).
Danvlas here has posted a method of getting around the AllowBypassKey property, but this method will stop 99.9% of all known users opening it directly.
Another enhancement of course would be to encrypt the table contents from within the delphi program (and ensure the routines are kept within the delphi source code), so that even if anyone gets hold of the file, they can't access the data from within Excel or linked tables in another access database.
John