Use an empty column. If your first number is in A1, use this formula
in the first row of your helper column:
=LEFT(A1,FIND("-",A1)-1)&RIGHT(A1,LEN(A1)-FIND("-",A1))
This will force the cell containing this formula to Text format. Copy
the formula down as far as you need, then copy all the new cells, and
without changing your selection, click Edit|Paste Special (Values).
Then you can copy the resulting values over your old column.
Mark Lincoln