Validating Data On Exit

I'm trying to figure out how to validate a field to make sure the users are not entering any blank spaces into the field. I've tried setting the mask up as "aaaaaa" but it still lets the user enter a blank space.

I've also tried building an event procedure to run on exit that reads:

Private Sub Barcode_Exit(Cancel As Integer)
Dim LPos As Integer
Dim LChar As String
Dim LValid_Values As String

'Start at first character in Barcode
LPos = 1

'Set up values that are considered to be alphanumeric
LValid_Values = " abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWX YZ+-.0123456789"

'Test each character in Barcode
While LPos <= Len(pValue)

'Single character in Barcode
LChar = Mid(pValue, LPos, 1)

'If character is not alphanumeric, return FALSE
If InStr(LValid_Values, LChar) = 0 Then
AlphaNumeric = False
Exit Function
End If

'Increment counter
LPos = LPos + 1

Wend

'Value is alphanumeric, return TRUE
AlphaNumeric = True
If AlphaNumberic = False Then
MsgBox "Barcode cannot contain blank spaces, only letters and numbers.", vbExclamation, "Barcode Field Value"
Else
End Sub

****
However the message box does not come up and the form will not then let you get out of that field.

Any ideas? I've been checking on this field by running a query and then manually correcting the errors but I would rather that the DB check and force correction at the time the data is entered.

Any and all help will be greatly appreciated.

Charis