It depends on the software you are using to do the export. If you are simply pasting into Excel then you can format the cells (in Excel) as Text first then paste any text you want.
If you are using Excel to IMPORT your data then that is a different issue. What program created the file you are trying to import and/or what is the file format? If you don't know, could you paste a few lines from your file?
As always, it helps to know which version of Excel you are using.
That said, you could try disabling calculations in Excel (though, honestly, I don't think it will work):
In Excel 2007:
Go to the Office button (the big colorful button in the upper-left corner of Excel) > Excel Options
On the Formulas tab change Workbook Calculation to Manual.
In Excel 2003:
Go to Tools > Options
On the Calculation tab select Manual.
Setting the workbook calculation to manual only affects the current workbook so in order for it to work your software that is exporting to Excel would have to work with an extant workbook in which you have disabled calculation.
For that matter, if your export software does work with an existing workbook then format the offending columns as text and save it before exporting to it.
My suspicion is that you may need to take an intermediate step and export your stuff to a .csv file then open that with Excel - that should do the trick.
Hope that helps...
Additional comments:
I'm afraid I'm unfamiliar with Quickbooks in any form. Is there any way to auomate QB so that it could prepend all occurrences of anything starting with a plus, minus, or equals sign with a single quote as it exports? (Thus =text would become '=text) Or perhaps strip them out? Are you exporting directly to a .xls file or a .csv file? If you are currently exporting to a .xls file try exporting to a .csv file. If QB doesn't bomb doing this, you will have a file that Excel can open. If this plan works and you get error message for the "formulas" you can press control-H in Excel and replace all occurences of = with '= which will fix your problem.
Let us know where that gets you...
I just reread your additional details and a thought occurred - when you say "it got to item #870" - what was "it"? QB or Excel? What exactly happens when it stops? Are there error messages? What do they say? If the problem is inside Excel then I forsee an easy fix. If the problem is occurring in QB then it's not really an Excel issue so much as it is a QB issue.
Thanks...