Just as an idea, you could try something more along the lines of
In your TextBox Before Accept
IF IsNumeric(Trim(Me.TextBox) THEN
Me.TextBox = TRIM(Me.TextBox)
Me.TextBox = 'ENTER ACCT NO"
This covers when a user decides to cut and past in an account number. With cut and paste you can often get a leading or trailing space. This removes those, checks to see if its a valid account number, then would go about its merry way.
As you are having them enter in an account number I am guessing you are doing other validations while your at it, so this would simply be one of those.
Trying to FORCE them to only press specific keys can cause an issue when uses can cut/paste, especially with the mouse. Best to let them do what they are doing and clean up afterwards since it catches more unusual cases.