Writing a formula in each cell wouldn't work because you would never be able
to enter your data without overwriting the formula. If you are not using any
Cell Formats on your worksheet, then you can try this Custom Cell Format...
0;-0;"-"
which, as written, assumes integer numbers in your cells. If you could have
floating point numbers, then you would need to change the zeroes to
something like this...
0.###;-0.###;"-"
where the number of # signs would equal the number of places to round your
values to (but note that whole numbers would display a trailing decimal
point). There is also a VB solution if you would like to consider that.