The easiest way I can think of is to change the number format of the cells in question, rather than using the conditional formatting tool.
Using the custom format in the format cells dialog box set the format of the cells to be:
then any positive values will always be displayed as 0, however the cell retains the actual value that has been input. Negative values will be displayed as their normal value. You can add additional formatting to the negative side of the format as per normal.
Hope this helps (and I must admit to being curious as to the reason why)