G'day Grover,
Depending on the nature of the data being entered, another method is to use Data Validation (found in the Data ribbon).
Select the column(s) you want to restrict the use of the "-" Select data Validation from the ribbon and again from the drop down, then select Whole number OR Decimal, then either select "greater than or equal to" and then put 0 in the Minimum box.
Select the Error Alert tab and enter a simple message such as "- not allowed, enter 0 instead".
Note- if you use negative numbers you will need to use the "between" option instead of "greater than or equal to 0" and apply limits that are outside of any possible range you would expect. This allows a "-" to be used with a number, but not by itself.
Cheers
shane