Excluding customers from Sales Analysis/Forecasting

Hi

We issue a number of samples to our sales reps which we do by setting them 
up as customers and raising a zero priced invoice to get the goods picked 
& despatched. This has the effect of adding the quantities to sales 
analysis, which in turn, goes through to the forecasting module and can 
give a false impression as to how well items are selling. 

Does anyone know of a way to exclude certain customers from being included 
in the sales analysis rebuild?

TIA

Angus