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There is no simple way at the table level to do this.
There are some features in V2010 that will allow the use of data-level macros; however, unless you are using a share-point site, until more understanding about these is available I'd avoid them for now.
So, IMHO, you have the following options:
Simple answer: If you need integers, then set the field to integers and do not allow the user to enter decimals.
A tad more effort: Set the field for integers and set the validation rule so that decimal entries are trapped and nice custom message pops up telling end-user to enter whole numbers only.
for a couple of bucks: Let the user enter double/long or what have you and in the query or in the report have a calculated field that converts the number to an integer.
and for the whole bankroll: Set up a form with a text box. in the before update event round the number to the nearest whole number, clear the user entry, and return the corrected value.