If the negative values all appear in the same column, you can do this easily
with filters. Select your entire table. Then, from the menu bar, Data >
Filter > AutoFilter. Click the drop-down in the column that has the negative
values and select Custom. In that dialog, choose 'is less than' '0' and
click OK. Now only the rows to be deleted should be visible.
Select (click in) a cell in the first such row, and ctrl + shift + down
arrow. Now all the cells with the negative values will be selected. Back to
the menu bar: Edit > Delete Row.
Now just turn off the filter: Data > Filter > AutoFilter.
HTH. --Bruce