One of the gurus will likely respond with a brilliant built-in Excel
function that does this automatically. Until they do, this work-around
will work. Assuming all your data is in column A, add a column, enter
this formula, and copy down to address the entire column of data:
=IF(RIGHT(TRIM(A1),1)="-",MID(TRIM(A1),1,LEN(TRIM(A1))-1)*-1,A1)

This formula checks each entry for the trailing negative sign, and
converts it from a string to a negative number; if the entry is
non-negative and numeric the formula returns the number. You can then
copy this column, paste it as values over your original data, and
delete the inserted column.