Controlling "ad hoc" queries

There are a large number of query tools available today. These tools allow end users to formulate their own queries and extract results of the query to a file, to a spreadsheet, etc.
I'm looking for some practical advice on how to keep this "ad hoc" access from crippling the
database. I'm especially concerned about end users creating queries that consume vast resources.

Has anyone out there successfully installed and supported an end user query tool that was actually used by end users? How did you prevent "monster" queries? How did you handle users formulating queries that returned misleading results? How did you /will you handle changes in the database schema?

Thanks for your help.

• Nat Brooks (nbrooks_at_bbn.com)