Select Case on Keypress

Hi.
I am creating a series of userforms with validations ie "number" fields should be numeric etc and I am using the following code to check this:
Private Sub Assignment_Field_NewEmployeeApplication_UF_KeyPress(ByVal KeyAscii As MSForms.ReturnInteger)
Assignment = Assignment_Field_NewEmployeeApplication_UF.Value
Select Case KeyAscii
'only numbers and - allowed
Case 45, 48 To 57
Case Else
MsgBox "Assignment Number must be Numeric"
End Select
End Sub
This works great however this still allows an incorrect character to be entered into the field.
I have tried simply deleting the last character with this code after the msgbox:
Assignment = Left(Assignment, Len(Assignment) - 1)
Assignment_Field_NewEmployeeApplication_UF.Value = Assignment
But this does not work as at that stage it seems the character is not in the field to be deleted. Stepping through the process does not show when the character is placed, but by running the procedure it places it.
Also - I would like to check the characters if the field was pasted into, I see there is "BeforeDropOrPaste" rather than keypress, but could you advise if I can somehow use a similar process to keypress (ie as above) for this type of event?
Any help is appreciated! Thank you