The obvious thing to do would be to tell everyone that my 500 GH/s pool is 400 GH/s, and reward everyone an 80% share on every hash. If you're sophisticated enough you might notice that my pool is mining blocks about 25% above what you'd expect, but how many data point do you need for that, and it's statistical, so I'll have runs of good or bad luck.

Another option is to dilute the pool of contributors, but again you might be able to detect that either I'm misreporting your hash rate, or the sum of all contributions doesn't line up.

Assuming I give you enough tools to figure any of this out.