I have just hit this problem, it's really annoying and there is no way I'm going to do it manually. Luckily I thought of this before I sent the form out, if you already have responses then this won't help (unless you send it out again).
To solve this problem I have:
- Set the question type to "Text"
- Under "Advanced Settings" I have selected "Regular Expression"
- In the next drop down box I have selected "Matches"
- In the text box I have entered ^[\d+]$
This regular expression forces the user to enter an open square bracket at the start of their response which then must be followed by at least one digit and it must be ended with a closed square bracket. For example  would be a valid entry. Note that this will only allow digits, if you just want to force the square brackets, you can use ^[.+]$ instead.
In the sheet, this will show as , so you will then need to remove the square brackets which can be done with a formula like =mid(A1,2,len(A1)-2) where A1 is the cell containing . Auto fill and formatting the cells as plain text should do the rest of what you'll need.