One way you could do it is to have them paste just the text each time. I wrote this code a little bit ago for this purpose. It recognizes if the text they are pasting is from Excel or another source. you could assign this to button or autoshape and then they can just click that button rather than CTRL V or right click then paste.
Code:
Sub PasteText()
On Error GoTo valueerror
ActiveSheet.PasteSpecial Format:="Text", Link:=False, DisplayAsIcon:= _
False
End
valueerror:
On Error GoTo clipboardempty
Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
:=False, Transpose:=False
End
clipboardempty:
MsgBox "Nothing on clipboard to paste."
End
End Sub