As this is a feature of Autonumber there is little you can do to change it.

There are two possible alternatives
[list=1][*]Change the primary key to a combination of fields where the contents will be unique. This will do away with the autonumber altogether.[*]Have a number field (not autonumber) and store the next key value in a table. When a field is added get the next key from the reference table, use it in your table, and increment the reference field for the next record to be added. When deleting the last record decrement the next key reference value. [/list=1]

While typing the above I realised the problem cannot easily be overcome when deleting records in the middle unless you hold a list of deleted record keys.

The simplest solution is to use point 1.