How To Prevent An Entry While Using A List

I Have A List Of Numbers From 0 To 10 With A Blank Cell As The First Entry In The List.now When I Am Using Thsi List Through Data Validation I Can Select The Options From Blank,o,1,2,3,4,5,6,7,8,9,10
But When I Type In Any Number,alphabet It Does Not Give Me An Error Or Prevent Me From Doing That?
Any Thing Which I Need To Do..?

The only reason I can think of is that you might be using a named range to define that validation. With a named range, a blank cell allows any entry.

The way around this is to use Oldchippy's method. Rather than define the named range, just select a a range.