Hi,
I'm not sure if this is what you need, but here goes....
You could try Data Validation - Press alt D L and select Text length from the Allow drop down list. select equal to from the Data drop down list and put 8 into the Length box. You can put an Error Alert message and an Input message saying "Use this format DD/MM/YY"
It wont stop anyone from entering dd-mm-yy but it will prevent dd/mm/yyyy.
Good luck
Ak
PS. I assume you have the cell formatted as DD/MM/YY