The default formatting for any new excel sheet is "General". Excel will then consider a dash (-) as being associated with dates and will thus format a cell as Date or Custom. You can re-format the column in question as text but it needs to be re-formatted before any data is entered or pasted into the column.
To test this method, open a blank excel worksheet and select any entire column ( just click on any letter above the column to completely high-light it). You will see that in the Home tab, numbers group, the formatting is set to "General".
Right click within the column and select "Format Cells" from the menu that appears. In the dialogue box that then appears, select "Text" from the category list. Click OK. Type in a few of your numbers to test.
Your numbers should then stay as is.
I hope that this helps.