Detect cheating in Microsoft Excel submissions
Figure out a way to generate data sets and results programatically so you can uniquely verify each student's assignment separately.  This would imply lots of effort initially but may payoff for later courses.
Use a single dataset and a single result set.  Ask them for their reasoning behind for achieving the most complex tasks of the assignment.  Use this to check for cheating.  No one person thinks exactly the same or describes something complex in the same manner.


My recommendation is for the latter.  Most jobs that can be done with tools like Excel will be automated in the near future.  The best that your students can learn is to reason (and articulate this effectively) and choose the appropriate tool/technique for the job.