drop down list accept data not from list at the time of copy paste

Hi all ,
I am using Ms Excel,
In my excel sheet i am creating drop down list(Combo box) using Data->Validation Option.
It is work nice at the time of select the list box , and i am try to enter values
not in the list , it shows error message.
but I am copy and paste the data is not contains the list , the list box accept
the pasted new data, i want how to avoid this problem,
when i am paste some new data is not contains the list then it shows error message , how can i do this .
eg : the list contains "one" , "two" , "three"
when i was type "four" then it shows error , but i am copy "four" from some other cell and paste the listbox cell then its accept ,
I hope i am explain my problem clearly,
plz tell some solution to avoid this problem,
Thanks in advance.

With Regards,
Ganesh Kumar.