limit user building a query

I am back with another question :wink:

I am building a batch class( derived from runbasebatch) where the user can configure his query ( possible by using showqueryvalues). My problem is : i have 2 datasources coupled with the outerjoin mode, but i dont want the user to be able to configure fields from the second datasource. Is this possible or shouldnt i waste my time looking for this ?