Hi Bond,
There are different ways to restrict customers however need to choose best way,
As per your requirement create authorization based reports this will solve the problem, if you restrict a query level , in future report maintain will become tough activity as we need to add new customers in all reports.
If you goto authorization,lest create one report that will fit for all and less maintenance,.
Please do share more about requirement so that you will get best approaches.
Thank you,
Nanda